feat(layout): opt-in RTL layout mirroring

[chiefcll]

Summary

Adds an inherited, opt-in rtl flag to nodes that mirrors layout horizontally — child x is measured from the parent's right edge, and left/right text alignment is reversed. Setting rtl on the root mirrors the whole app; an explicit rtl: false opts a sub-tree back out.

Scope: this is RTL layout structure only (rails/nav mirroring, alignment). Bidirectional text reordering (Hebrew/Arabic glyph order) is a separate opt-in follow-up (Phase 2) and is intentionally out of scope here. Until that lands, rtl should be documented as "layout mirroring", not full RTL.

How it works

• New rtl: boolean | null prop (null = inherit, the default), exposed via INodeProps/INode.
• _ownRtl (local override) + resolved _rtl on CoreNode; inheritance resolves top-down through a new UpdateType.Direction pass that reuses the existing dirty-flag traversal. The root is resolved eagerly in its setter (it's driven by Stage.drawFrame, not update()).
• The mirror (lt.tx += parentW − 2·x − w·scaleX) is applied as a temporary shift/restore of the local transform during the world-transform compose, so the cached transform stays pristine and isn't double-mirrored next frame.
• Hot-path cost: LTR apps pay a single parent._rtl === true check per node; no bundle weight worth gating.
• textAlign left/right flips via a shared resolveTextAlign() used by both Canvas and SDF renderers and text-node containment; SDF layout cache key now includes rtl. Text nodes resolve direction before generating layout, so a change reflows in the same frame (no flash).

Caveat

Mirroring requires a known parent width (w). A parent without w lays out LTR (matches the upstream Lightning behavior).

Test plan

• 11 new unit tests (CoreNode.test.ts): inheritance, explicit override, mirror math, scaleX, LTR/zero-width no-mirror cases, local-transform restoration.
• Full unit suite green (254 passing).
• tsc --build clean; Prettier clean.
• New visual-regression example examples/tests/rtl-layout.ts with a CI-certified snapshot generated via Docker. Visually verified: positions mirror under an RTL parent, child markers mirror by each box's own resolved rtl, the rtl: false override works, and RTL text right-aligns.

🤖 Generated with Claude Code

[elsassph]

Maybe could have gone with if (rtlOrigTx !== 0) and drop the bool