docs: 1.16.0 release notes, and publish notes as the GitHub release body

[craigmcchesney]

Prepares the 1.16.0 release: adds the master release notes, wires the release workflow to publish them, and brings the README up to date.

Publishing the notes (ci: commit)

Follows the pattern established in dp-grpc. release.yml reads doc/release-notes/rel-<version>.md and publishes it as the release body via body_path, and a Verify release notes exist step fails the job when the file is not on the tagged commit.

The check runs immediately after the version is extracted from the tag rather than late in the job. action-gh-release would catch a missing body_path on its own, but only after the three sibling-repo JARs have been downloaded and the tarball assembled — this repo's expensive work is all upstream of the publish step, so checking first turns a minutes-late failure into a seconds-late one.

doc/developer/release.md gains the ordering rule this creates: the notes must be merged to main before the rel-* tag is pushed, because the tag is what the workflow builds from.

The notes themselves (docs: commit)

Organized by feature rather than by repo. The two headline APIs — sample status, and the DataSet/Annotation modernization — each span all four child repos, so a per-repo structure would scatter them. Each section links to the child notes that carry the detail.

Six features covered: Sample Status API, DataSet and Annotation API modernization, query performance, the new metrics framework, the schema migration mechanism, and the desktop app's deployment mode. The last two are not new API surface, but they are why this upgrade is not a drop-in binary replacement, so they get their own sections.

Two structural choices worth a look during review:

• A "correctness fixes worth knowing" section collects the defects that returned wrong answers silently rather than erroring — querySamples omitting PVs on large pages, the desktop app destroying stored Calculations on an unrelated edit. Operators may need to re-examine results from earlier releases, which is easy to miss if these are scattered across six feature sections.
• The upgrade checklist leads the document, because a partial service stop during the migration produces silent wrong answers rather than an error.

README

Adds the release-notes table and a v1.16 milestone entry in the existing house style.

The v1.16 road map block becomes v1.17: two of its Python items did not ship (ingestion API interface, bucket-oriented query interface) and are already retargeted to 1.17 on the project board, so they carry forward rather than disappearing. The FY27 data-subscription bullet is dropped as a duplicate of the 1.17 one.

Also fixes four anchors that were already broken on main — a TOC link missing the heading's mldp- prefix, and three v1.0/v1.1/v1.3 milestone links to a "Data Platform API" section that no longer exists, retargeted to its successor "gRPC API" with the link text updated to match.

Review follow-ups (commits 3 and 4)

Applied after a fact-check of the master notes against all four child repos' rel-1.16.0 notes.
Every cross-repo link and anchor was re-validated after the edits; all resolve.

Attribution errors. The metrics section was labeled data-platform #212 against a dp-service
issue URL — data-platform#212 does not exist. The query performance header listed #257, which
is not a change in this release: it is the "option 0" workaround ticket that this release makes
obsolete, and it is now described in the body as retired. #275 was cited in the body but
missing from the header, and is now listed.

Upgrade checklist gaps. The checklist is the operationally risky part of the document, and
compression had dropped four things the child notes carry:

• No fallback for a site that cannot stop everything at once — the most likely real deviation
  on a 24/7 archive. dp-service gives a specific safe ordering (ingestion first, but stop or
  upgrade query and annotation before it migrates), now in step 3, along with the write-side
  reason for stopping ingestion.
• Step 10 stated the empty-criteria rule as universal. ConfigurationSelector is the
  documented exception and rejects an empty list. dp-grpc wrote that section specifically so the
  rule would not be learned backwards.
• Step 4 gave only a range for the migration window. dp-service says to budget it from a
  read-only measurement of the archive; the SLAC runbook has the query.
• The Python grpcio floor was missing (new step 7). It raises at import, not call time,
  and a fresh install never sees it — so it surfaces first in a pinned environment.

Smaller: CalculationsDataFrame's frame submessage (needed by anyone fixing compile errors),
the retired index shapes named with the runbook that lists them, the migration-disable flag, and
three restored omissions.

The dry-run rehearsal path is now in this repo too. The body_path wiring added here could
previously only be exercised by cutting a real release — the exact failure mode CLAUDE.md warns
about. release.yml gains the workflow_dispatch + dry_run pattern from dp-desktop-app.

I rehearsed it: run 35156190426
— all three sibling JARs downloaded, tarball built and checksummed, notes verified and
NOTES_PATH resolved to the real document, and only Publish GitHub Release skipped. No
release was created.

release.md also regains the tag-ordering constraint dropped in the rewrite. It matters more
here than in the sibling repos: this job downloads their JARs, so tagging this repo before their
releases exist fails the download.

Notes for the reviewer

• Cross-repo links are absolute and pinned to rel-1.16.0, per Release notes: enforce absolute, tag-pinned cross-links across all five repos #98. My first draft used a relative link to CLAUDE.md; Release notes: enforce absolute, tag-pinned cross-links across all five repos #98 explains why that 404s in a release body, and it is fixed here. All ~30 cross-repo deep links were validated against the actual headings in the child notes — zero broken.
• Do not tag rel-1.16.0 until dp-service's release is published. Its tag exists but CI was still running at the time of writing, and the tarball job downloads dp-service-1.16.0.jar from that release.
• The notes were drafted from the four child repos' rel-1.16.0.md files and the 1.16 items on project board 2. Emphasis and feature framing are worth a careful read — that is the part a validator cannot check.

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