fix(wasm,renderer): offset iteration budget by SA skip and floor slice height

[rowan-m]

Summary

1. Offset calc_iter by Series Approximation skip_iter (wasm/src/lib.rs):

   • At deep views inside narrow valleys/clovers (e.g. z = 16.681, skip_iter = 21,311), up to 80% of the initial search_limit (27,018) is consumed by the shared pre-rebase prefix before orbits in the viewport even begin to diverge from one another.
   • Because search_anchor stops as soon as the centre candidate reaches search_limit, Phase 2 of the anchor search was skipped and calc_iter capped out at 40,527 — leaving only ~19k post-skip iterations and clipping the central 4-lobed clover (41k–107k iterations) to solid black.
   • Offset calc_iter by skip_iter, advance the reference orbit through that post-skip budget, and when an anchor that survived search_limit subsequently escapes during the + skip_iter extension, scale calc_iter to at least orbit.n + 3 * skip_iter.
   • Shallow views (skip_iter == 0) and periodic bulb interiors (!orbit.escaped) are unaffected.

2. Floor progressive slice height at a 32-row hardware tile strip (src/Renderer.js):

   • When effectiveIter is large (86,197 post-skip iterations at z = 16.681), a single GPU thread's sequential loop latency (~50–67 ms) exceeds TARGET_SLICE_MS (32 ms).
   • Without a minimum slice height, nextRows = ceil(prevRows * 32 / elapsedMs) decayed exponentially (52 -> 21 -> 11 -> 6 -> 4 -> 2 -> 1 row), starving the GPU's parallel shader lanes while still paying the full single-thread serial latency and full-screen post-pass blit on every 1-row slice (27.4 s across 411 slices on Pixel 11).
   • Floor _sliceRows at MIN_SLICE_ROWS (32 rows, one 32x32 mobile TBDR hardware tile strip, capped by remaining), cutting the full-resolution 1078x2008 render at iter = 107,508 on the Pixel 11 from 27.4 s (411 slices) to 3.19 s (48 slices, 8.6x faster).

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