chore(deps): upgrade typescript 5.8.3 to 5.9.3

[dtoxvanilla1991]

Summary

Clean replacement for #124 (which has merge conflicts against the current main).

Upgrades typescript from 5.8.3 to 5.9.3 (minor version bump).
No migration required. Zero type errors (tsc --noEmit clean). All 51 tests pass.

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Is this a breaking change?

No migration needed for this codebase. Here is the full Breaking Changes Specialist assessment:

TS 5.9 breaking changes and their impact on this repo

Breaking Change	Impact on this repo
importsNotUsedAsValues and preserveValueImports fully REMOVED (error TS5102)	Not affected - neither option is in tsconfig.json
lib.d.ts ArrayBuffer/Buffer hierarchy change (Buffer no longer assignable to Uint8Array<ArrayBufferLike>)	Not affected - tsc --noEmit passes with 0 errors
Type argument inference tightening (potential new errors in generic code)	Not affected - tsc --noEmit passes with 0 errors
Removed legacy options: target: ES3, keyofStringsOnly, charset, out, etc.	Not affected - tsconfig.json uses none of these

New features in TypeScript 5.9 (additive, no action required)

• import defer syntax support for deferred module evaluation
• --module node20 stable option
• Summary descriptions in DOM APIs
• Expandable hover tooltips (preview, editor-only)
• Performance optimizations (type instantiation caching, ~11% speed-up in large projects)

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Changes

• package.json: specifier bumped ^5.4.5 to ^5.9.3 (enforces 5.9.3 as the floor)
• pnpm-lock.yaml: lockfile updated - typescript resolves to 5.9.3

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Supersedes: #124

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/upgrade/typescript-5.9.3

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