feat: speak display math + switch TTS rewrite to gemma4:e4b

[elimelt]

What

Two changes to the TTS reader:

1. Speak display math. Quartz emits $$...$$ as a <span class="katex-display"> that is a sibling of paragraphs, not inside one, so the block selector (p, li, dt, dd, h1-h6) never collected it. Verified across the built site: all 402 katex-display spans sat outside any block element — every standalone equation on every note was silently dropped from the reading. Fix: add .katex-display to BLOCKS. The existing nested-block filter and the data-tex path in textOf handle the rest; each display equation becomes its own chunk carrying its full LaTeX. This surfaced 533 pure-math blocks corpus-wide that were previously inaudible.

2. Switch the rewrite model to gemma4:e4b (think: false). Eval vs the shipped qwen2.5-coder:7b (both with the S3 prompt):

	gemma4:e4b	qwen2.5-coder:7b	gpt-oss:20b (low)	gemma2:2b
Semantic suite	18/18	18/18	18/18	11/18
Residual LaTeX (15 real blocks)	0/15	1/15	0/15	—
Warm rewrite	~2.5s	3.4–4.0s	7.8s	8.7s
Cold prompt eval (~1.6k tok)	28s	49s	—	—
Timeline wall (120s audio)	122.4s	127.7s	—	—

CACHE_VERSION bumped to 6 so stale qwen rewrites regenerate.

Sample readings (display equations, gemma4:e4b)

• t_h \leq t_{input,i} \leq T_{clk} - t_s ~ \forall i → "t h is less than or equal to t input i is less than or equal to T clock minus t s for all i"
• T = \frac{80 \times 10^9 bytes}{3.35 \times 10^{12} bytes/s} \approx 23.9 ms → "T equals eighty times ten to the nine bytes over three point three five times ten to the twelve bytes per second approximately twenty three point nine milliseconds"

Verification

• Timeline replay on a display-math-heavy note (static-timing-analysis, 24 chunks, 11 LLM-bound after the fix), warm cache: 0.7s to first audio, one 1.6s stall, wall 188.9s for 186.5s of audio — near-realtime with double the previous LLM load.
• Cold-cache replay shows one 13s stall from the one-time prompt eval; in the browser the urgency-skip path covers this (deterministic rendering plays immediately while the quality rewrite fills the cache).
• Build passes; emitted bundle parses and contains the new selector, model, think: false, and cache v6.