fix: stop image map area ids leaking, without an unbounded obarray

[sudo-human]

Problem

svg-margin--cell-area tags each clickable/help hot-spot with a fresh make-symbol area id on every render. Emacs looks images up in its cache with equal, and two uninterned symbols of the same name are never equal — so a byte-identical composite image rebuilt on the next render is a cache miss and is retained for image-cache-eviction-delay (300s by default) instead of reused.

On a buffer with a :help/:action indicator on many lines — e.g. diff-hl on a merge-conflict file, where nearly every line is a hunk — a frequent re-render (idle timer, flydiff, after-change) regenerates hundreds of never-deduped SVG images repeatedly. The image cache climbs steadily until eviction, which presents as a memory leak.

Why it's fiddly — three conflicting constraints on the area id

1. Must be a symbol — Emacs composes the click event by combining the id with the mouse event (e.g. [ID mouse-1], per Image Descriptors in the Elisp manual).
2. Must be distinct per cell — a shared id makes mouse-highlight treat two hot-spots as one and stop re-firing help-echo between them.
3. Must be stable across renders of the same cell — otherwise the composite image is never equal and the cache never hits (the bug above).

make-symbol satisfies (1) and (2) but not (3). Interning the name — even in a private obarray — satisfies all three, but because the name embeds the buffer position, it accumulates a permanent symbol for every position ever drawn. A long session that shifts positions (typing above the indicators, or a growing file) then leaks without bound instead. Interned symbols are never GC'd.

Fix

Memoize an uninterned symbol per cell in a small buffer-local table that is rotated each render (svg-margin--area-rotate, svg-margin--area-id):

• A cell still present reuses its symbol from the previous generation → image stays equal → cache hit.
• A cell that disappears is left only in the retired table, dropped at the next rotation, and garbage-collected.
• The live set is bounded by the cells of the last two renders — nothing accumulates.

This keeps all three constraints: symbol type (clicks work), distinct per cell (hover works), stable across renders (cache hits), and now bounded (no leak either way).

Verification

Measured the live area-symbol set across editing patterns (before → after):

Scenario	fresh make-symbol	private obarray	this PR
stable file, 100 renders	leaks image cache	ok	bounded, cache hits
delete lines from end	leaks image cache	ok	bounded
growing file (new positions)	leaks image cache	unbounded obarray	bounded (live ≤ 2 gens)
edit above indicators	leaks image cache	unbounded obarray	bounded (live ≤ 2 gens)

Tests

Added tests asserting the id is a symbol, uninterned in the global obarray, distinct per cell, stable across renders (including across a rotation), and bounded under position churn.

Full suite: 60 passed, 1 skipped (image-builds, needs a GUI/librsvg), 0 unexpected. Byte-compiles with no warnings; checkdoc clean.

[sudo-human]

Hey, this is a vibe coded fix with claude. I don't really have much elsip knowledge apart from write my config 😅