Resolve json tag names through encoding/json

[FumingPower3925]

Closes #126. main is red on Go 1.27, which joined the version matrix the day it went stable — the matrix derives from go.dev.

The break

Go 1.27 rebuilt encoding/json on top of encoding/json/v2 (type RawMessage = jsontext.Value) and changed which json: tag names it accepts. stdocs carried a copy of the old rule in isValidTagName, so the documented key and the wire key diverged:

tag	wire key ≤1.26	wire key 1.27	stdocs documented
json:"🚀"	Emoji (field name)	🚀	Emoji ❌
json:"a\"b"	X (field name)	truncated to a	X ❌

The behaviour follows the toolchain, not the go directive, and GODEBUG=jsonv2=0 does not restore it. Since the supported floor is Go 1.24, the same source has to produce the right key on 1.24 through 1.27.

The fix

Stop mirroring the rule; ask encoding/json for the answer. parseJSONTag resolves a tag by marshalling a synthesized field that carries it, so the answer comes from the same encoding/json that will marshal the caller's values. Cached per tag.

The tag is probed under two differently named fields. A tag that really names a key yields that key twice; one encoding/json ignores yields each probe its own field name, and the disagreement signals the fallback. This matters: a single probe has to reserve a name to signal fallback, and a real tag can always spell it — a field tagged json:"StdocsWireProbe" would then be documented under its Go name while the wire carried StdocsWireProbe, and an embedded field would additionally be flattened, because the empty name also drives embedded and tagged in inspectField. Both shapes are regression-tested.

Deliberately scoped to name resolution. The BFS embedding/dominance logic from #70 still decides conflicts and flattening, and the tests confirm the composition is right: on 1.27 two tags that truncate to the same key collide, encoding/json drops both, and so does stdocs.

Tests

TestInvalidJSONTagNames pinned the rule that just changed, so it is replaced by TestTagNamesTrackEncodingJSON — a table that marshals each case with the real encoding/json and asserts the schema documents exactly the keys that appear, in both directions. It covers the old test's tags plus embedding, an outer tag shadowing a promoted one, a name that collides once truncated, and a tag spelling a probe's own field name.

Because it compares against a live marshal rather than a fixed expectation, it passes on both sides of the change instead of encoding either toolchain's answer.

Verification

Everything below was run under both GOTOOLCHAIN=go1.26.6 and go1.27.0:

• gofmt/vet/build, full suite, -race, and the nested YAML module — green on both; golangci-lint 0 issues.
• The fuzzer, whose corpus already crosses odd tags with odd field types: 3.6M execs clean on 1.27.
• Negative control: reverting just the probe to the single-field form makes both sentinel cases fail on both toolchains, so the new tests genuinely catch the bug rather than passing vacuously.

An adversarial review pass over this change found the forgeable-sentinel bug above before it shipped; it also turned up two wrong-contract shapes that predate this work and reproduce on both toolchains, filed separately as #127.

Note

This unblocks #125 (the Scalar/Swagger bump), which is verified but sitting on red CI for this reason. It rebases onto this and releases as v0.9.3.

[FumingPower3925]

Re-triggering CI: the earlier runs were lost to the GitHub Actions outage (major outage 15:11Z–~16:0xZ), not to this change.

[FumingPower3925]

Added a second commit: the Lint job pinned golangci-lint 2.12.2, whose bundled typechecker predates Go 1.27 and rejects the 1.27 standard library outright (could not import math/rand/v2 ... method must have no type parameters) — two typecheck errors in crypto/internal, none in this repo. 2.13.0 added Go 1.27 support; both pins now move to 2.13.1.

That was the last red job: Test, Coverage, Spec validation and Go 1.27.0 all passed on the previous run.

(The earlier runs on this PR were lost to the GitHub Actions outage on the 26th, not to this change.)