feat!: rename Items to Values (#12)

[millerjp]

Summary

Renames SyncMap.Items() []V to SyncMap.Values() []V to match Go convention (maps.Values in stdlib 1.23). Pre-v1.0 breaking change; zero known importers under the axonops module path; no deprecation shim.

Closes #12.

Diff

• syncmap.go — method rename + godoc first line + internal local var.
• doc.go — package overview now lists Keys, and Values instead of Keys, and Items.
• syncmap_test.go — TestItems → TestValues; local vars renamed; subtest names and assertions unchanged.

Verification

$ git grep -nE '\bItems\b' -- . ':!docs/' ':!.issue-drafts/'
(none)

$ go doc github.com/axonops/syncmap Values | head -5
func (m *SyncMap[K, V]) Values() []V
    Values returns a slice of all values present in the map at the moment of the
    call. It runs in O(n) time.

$ go doc github.com/axonops/syncmap Items
doc: no symbol Items in package …

$ make check
… (14 gates green, coverage 100%, no vulnerabilities)

Agent gates

• Plan: single-commit scope, two source files + test.
• code-reviewer: caught a missed reference in doc.go:30 ("Keys, and Items"); fixed in the same commit.
• security-reviewer: skipped — no concurrency or type-assertion semantics touched (rename only).
• performance-reviewer: skipped — identical machine code, no benchmark delta possible.
• go-quality: 14/14 green (confirmed via make check).
• commit-message-reviewer: PASS.

Out of scope

• CHANGELOG.md entry — deferred to docs: add README, CHANGELOG, NOTICE, SECURITY, and LLM docs bundle #17 (CHANGELOG owner). docs: add README, CHANGELOG, NOTICE, SECURITY, and LLM docs bundle #17 must include this rename under the v1.0.0 Breaking section.
• README API table update — deferred to docs: add README, CHANGELOG, NOTICE, SECURITY, and LLM docs bundle #17.
• Example / benchmark / BDD feature renames — those files don't exist yet; the rename will land naturally with test: add benchmarks, runnable examples, and bench-regression infrastructure #15/test: add BDD (godog) suite and fuzz targets #16 which will create them against the new name.

Test plan

• CI green end-to-end
• pkg.go.dev search for importers of github.com/axonops/syncmap confirms zero importers (no breakage in the wild)