perf(#312): index argv once in parseNamedArgs (#392)

parseNamedArgs re-scanned argv with indexOf/includes once per flag —
O(flags * argv) — on the command-dispatch hot path (24 call sites across
gsd-tools + init/state/validate routers). Build a first-index Map of argv
tokens in a single pass and use it for the flag lookups, dropping it to
O(argv + flags). Semantics are identical: firstIndex.get(t)??-1 === indexOf(t),
firstIndex.has(t) === includes(t); first-occurrence-wins and the value-token
rejection are preserved. Adds the first behavior-lock tests for the module.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/312-arg-projection-single-pass.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 312
+---
+Index argv once in parseNamedArgs instead of re-scanning per flag (O(flags*argv) -> O(argv+flags)) (#312).