test: add BDD (godog) suite and fuzz targets

Summary

Add a Godog BDD suite covering every public method and every concurrency scenario the library guarantees, plus go test -fuzz targets for Load/Store round-trips and random concurrent operations. Re-enables the bdd-strict-mode-guard CI job that was deferred in #8.

Requirements

1. tests/bdd/ tree:
   • tests/bdd/features/syncmap.feature — Gherkin scenarios covering: Load (present / missing / zero V), Store (new / overwrite), LoadOrStore (absent / present / contention), LoadAndDelete (present / missing / zero V), Delete (present / missing / idempotent), Range (all / early return / concurrent writes), Len (after store / after delete), Map (snapshot independence), Keys (match / empty), Values (match / empty), Swap (absent / present / concurrent), Clear (empty / populated), CompareAndSwap (match / mismatch / missing / contention), CompareAndDelete (match / mismatch / missing).
   • tests/bdd/steps/syncmap_steps.go — step definitions; thin wrappers that translate Gherkin to SyncMap calls.
   • tests/bdd/suite_test.go — godog TestSuite{} entry with Strict: true explicitly set (satisfies bdd-strict-mode-guard).
   • All files carry Apache 2.0 header; build tag //go:build bdd.
2. Fuzz targets in a new syncmap_fuzz_test.go (package syncmap_test, Apache header):
   • FuzzLoadStore — random byte keys + values, verify Load returns what was Stored.
   • FuzzConcurrent — random interleaved Store/Delete/Load operations across multiple goroutines; asserts no panic under -race.
3. makefile-targets-guard stays satisfied; make test-bdd is already in place.
4. CI: re-enable bdd-strict-mode-guard job in ci.yml (ported verbatim from mask, now that tests/bdd/ exists).
5. No @wip / @skip / @pending / @todo tags anywhere in feature files (enforced by bdd-strict-mode-guard).
6. godog and goleak dependencies added to go.mod if not already present. Both are test-only.

Acceptance Criteria

1. tests/bdd/features/syncmap.feature exists with ≥1 Scenario per public symbol.
2. tests/bdd/steps/syncmap_steps.go exists and registers step definitions for every phrase used in the feature file.
3. tests/bdd/suite_test.go contains exactly one godog.TestSuite{…} construction with Strict: true on a line by itself (grep-able).
4. make test-bdd exits 0, runs every scenario, reports zero failures and zero undefined/pending/skipped steps.
5. go test -fuzz=FuzzLoadStore -fuzztime=30s ./... runs without panics.
6. go test -fuzz=FuzzConcurrent -fuzztime=30s ./... runs without panics under -race.
7. bdd-strict-mode-guard job in ci.yml is enabled and passes on this PR.
8. make check passes; CI on the PR green.
9. Every new .go file carries the Apache 2.0 header.
10. No AI-attribution tokens in any new file.

Testing Requirements

• make test-bdd green.
• go test -fuzz short runs (CI runs with explicit -fuzztime) green.
• CI bdd-strict-mode-guard green.
• make check green.

Documentation Requirements

• BDD features serve as behavioural documentation — they replace the "what does the library do?" prose narrative.
• README reference to the BDD location is added in feat: add top-level CompareAndSwap and CompareAndDelete functions #14.

Dependencies

• Blocked by: test: rewrite unit tests to external package with testify/goleak; coverage ≥95% #10 (tests scaffolding), chore: align CI, release, and tooling to axonops/mask pattern (#8) #9 (Swap/Clear methods must exist to be BDD-tested), test: rewrite unit tests to external package with testify/goleak; coverage ≥95% #10-CAS (same reason).
• Blocks: release v1.0.0 (BDD is "the contract" per CLAUDE.md).

Labels

• testing
• P1