move render cache ownership

[forketyfork]

Solution:

• Move grid render caching into a renderer-owned RenderCache keyed by session, and derive redraw decisions from SessionState render_epoch.
• Replace direct dirty mutations with SessionState.markDirty to make invalidation explicit and decouple renderer from session state.
• Reuse a per-frame SessionUiInfo buffer and add a short-lived foreground-process cache for UiHost snapshots to reduce per-frame churn.
• Update architecture docs and engine plan notes to document cache ownership and epoch-based invalidation.

[Copilot]

Pull request overview

This PR refactors render caching by moving ownership from session state to the renderer, replacing boolean dirty flags with an epoch-based invalidation mechanism.

Changes:

• Moved grid render cache from SessionState to a new RenderCache structure in the renderer
• Replaced SessionState.dirty boolean with render_epoch counter and introduced markDirty() method
• Added per-frame SessionUiInfo buffer reuse and foreground process caching to reduce allocations

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/session/state.zig	Removed cache texture fields and dirty flag; added render_epoch and markDirty()
src/render/renderer.zig	Added RenderCache struct with per-session entries tracking epochs and textures
src/main.zig	Integrated RenderCache, reused SessionUiInfo buffer, added ForegroundProcessCache
docs/engine_plan_correction.md	Updated to reflect renderer-owned cache resolution
docs/architecture.md	Documented epoch-based invalidation and cache ownership

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