Add symbol contain fitting and local viewbox bounds (#4698)

[Abse2001]

Custom symbols need a way to preserve their proportions during sizing and define their own source bounds. Adds optional fit="fill" | "contain" | "stretch" and viewbox={{ minX, maxX, minY, maxY }} to the public symbol props and runtime validation, enabling tscircuit/core#4706.

contain requests uniform scaling; CSS-style fill and explicit stretch retain independent scaling. Viewbox bounds use the local symbol coordinate space (mm, +X right, +Y up), accept numbers or distance strings, and require maxX > minX and maxY > minY. Existing defaults are preserved. Generated API documentation is included.

Follows Seve’s fit-mode review and min/max bounds review.

Validation: all 683 tests, full TypeScript checking, package/declaration build, and all four documentation generators pass. The current PR head has the same Git tree as the validated fix. Hosted test, typecheck, formatting, and package-preview checks are green.

Includes the fix already merged in #943. This PR targets main and now contains both the repro tests and their implementation, with ordinary passing assertions. After it lands and a props release is published, core#4706 can replace its preview dependency with that published version.

[tscircuitbot]

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PR Rating: ⭐⭐
Impact: Minor

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