feat: eliminate @ prefix stripping round-trip in locals

[rohal12]

Summary

• Store locals without the @ prefix internally, removing the strip-on-read / add-on-write round-trip
• Remove stripLocalsPrefix() function and all callsites
• Simplify useMergedLocals(), executeMutation(), and mutating macro components

Closes #24

release-npm

Test plan

• npx tsc --noEmit passes
• All 656 unit/integration tests pass
• All 177 e2e tests pass

🤖 Generated with Claude Code

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	73.44%	1778 / 2421
🔵	Statements	72.85%	1954 / 2682
🔵	Functions	67.35%	328 / 487
🔵	Branches	63.5%	931 / 1466

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/execute-mutation.ts	78.57%	50%	100%	78.57%	23-24, 29
src/components/macros/Button.tsx	91.66%	75%	100%	91.66%	27-30
src/components/macros/Do.tsx	85.71%	100%	100%	85.71%	20
src/components/macros/For.tsx	83.33%	66.66%	80%	83.72%	36, 47, 50-52, 84, 108-114, 132-138, 160-166
src/components/macros/MacroLink.tsx	71.05%	55%	80%	72.97%	34, 49, 54-67, 100-102
src/components/macros/Set.tsx	77.77%	100%	100%	77.77%	20-24
src/components/macros/VarDisplay.tsx	78.26%	72.22%	100%	77.27%	37-42
src/components/macros/WidgetInvocation.tsx	70.96%	40%	62.5%	70.37%	47-65, 90, 92, 114, 128
src/hooks/use-merged-locals.ts	100%	100%	100%	100%

Generated in workflow #78 for commit 5a13e94 by the Vitest Coverage Report Action