Add Luax.set-string-global

[carpentry-agent]

The set-*-global family in Luax had int, float, and bool variants but was missing string. This meant Luax.set-string-global didn't exist despite Luax.get-string-global, all set-*-field variants, and the Lua.set-string-global docs all pointing users toward it.

Adds (luax--def-set-global string Lua.push-carp-str) alongside the existing three variants, plus tests for the new function.

Also updated existing tests that used Lua.set-string-global to use Luax.set-string-global where appropriate.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

CI passes on both macOS and Ubuntu. No local Carp compiler on this machine — verified via CI only.

Findings

No issues found. This is a clean, mechanical addition that follows the exact pattern of the existing set-int-global, set-float-global, and set-bool-global variants.

• lua.carp:526 — (luax--def-set-global string Lua.push-carp-str) mirrors the set-*-field family at line 546 which already has a string variant using the same pusher. Consistent.
• The macro at line 42 expands to the same logic as the hand-written Lua.set-string-global (line 299), but scoped to Luax — no naming conflict.
• Tests are thorough: basic set/get round-trip plus a cross-boundary test that sets a global from Carp and reads it back through a Lua expression (do-string).
• The two test migrations from Lua.set-string-global → Luax.set-string-global (lines 249, 279) are correct — those tests exercise Luax.get-*-global and should use the Luax setter for consistency.

Verdict: merge

Straightforward gap-fill. Code is correct, tests are good, CI is green.