[thekevinbot]

Slice D of #1072/#1111: the last piece of extension resolution both SDKs still
ported from one another — the programmatic extensions=[{path}] probe — moves
into the core, and the binding exports it made necessary go away.

dirsql::extension_resolution::plan_extension_path classifies one path the same
way the config planner classifies a config entry: a path-looking value is a
Literal (made absolute against base only when asked), a bare name is a
Package with a shadow probe. Python's resolve_extension_path and
TypeScript's resolveExtensionPath are now one call to the planner plus the
existing plan_entry_path / planEntryPath, so each host keeps only the
package lookup it alone can do (importlib.util.find_spec / require.resolve).

is_bare_name / isBareName were that probe's only callers; both bindings drop
them in favor of plan_extension_path / planExtensionPath.

Resolved paths and error messages are unchanged, so no migration fragment.

Fixes #1122.

Parity

Restores parity rather than drifting: both SDKs move onto the same core
contract, and Rust stays file-path-only by design (the #227 carve-out).
PARITY.md's "Where the planning lives" paragraph is updated.

Manual verification

Built the packages/rust/tests/fixtures/testext cdylib, installed it as a fake
package, and resolved it by bare name through both SDKs:

# Python (programmatic + config entry), PYTHONPATH=<tmp>/pkgroot
programmatic bare name -> [{'a': 42}]
config bare name       -> [{'a': 42}]

# Node (programmatic), package in a resolvable node_modules
programmatic bare name -> [ { a: 42 } ]

E2E Verification

• Ran e2e suites locally for every affected SDK
• Python SDK e2e: pass (37 passed)
• TypeScript SDK e2e: pass (17 passed)
• Rust core e2e (if applicable): pass (cargo test --workspace --features cli via preflight)
• packages/python/e2e-attestations/<branch>.json receipt written if packages/python changed (just e2e-attest-python)
• packages/ts/e2e-attestations/<branch>.json receipt written if packages/ts changed (just e2e-attest-ts)
• Command(s) run: just test-e2e, just e2e-attest-python, just e2e-attest-ts, just preflight
• Result summary: all green; just preflight reports 0 failing pairs.

Changelog / Migrations

• Changelog fragment added under <root>/<pkg>/changelog.d/ for each changed package (rust, python, ts)
• Migration fragment added under <root>/<pkg>/migrations.d/ (not required — internal refactor, no observable change)

Red/green

600349e pushed the core contract with the implementation stubbed; CI was
observed red for the right reason (not implemented: plan_extension_path, 4
failing tests in dirsql Rust CI). c053177 implements it.