terminal: bound page map probe lengths

#13292

Page maps previously allowed a 100% load factor. Once live entries
and tombstones filled every slot, a missing-key lookup or insertion
could scan the entire map.

Reserve 20% of each offset hash map as insertion headroom and track
that budget separately from the live count. Port the allocation-free
in-place rehash from Zig HashMapUnmanaged so canonical insertion can
rebuild fragmented probes and restore tombstone-exhausted headroom
without growing the page. Assumed-capacity insertion now applies the
same guard, preventing the headroom counter from wrapping.

Pass the exact requested hyperlink count into map layout before
load-factor scaling, avoiding a redundant power-of-two rounding step.
Screen-level recovery now grows only when live hyperlinks actually fill
usable capacity.

ReleaseFast terminal benchmarks compare main with the combined map
changes:

| workload | before | after | speedup |
|---|---:|---:|---:|
| map churn | 1.253 s | 12.5 ms | 100x |
| full OSC 8 stream | 3.576 s | 75.6 ms | 47x |
| clear and redraw | 299.1 ms | 118.3 ms | 2.5x |

Co-authored-by: Tim Culverhouse <tim@timculverhouse.com>

src/terminal/hyperlink.zig

@@ -20,7 +20,7 @@ pub const Id = size.HyperlinkCountInt;
 // The mapping of cell to hyperlink. We use an offset hash map to save space
 // since its very unlikely a cell is a hyperlink, so its a waste to store
 // the hyperlink ID in the cell itself.
-pub const Map = AutoOffsetHashMap(Offset(Cell), Id);
+pub const Map = AutoOffsetHashMap(Offset(Cell), Id, 80);
 
 /// A fully decoded hyperlink that may or may not have its
 /// memory within a page. The memory location of this is dependent