repr: back RowArena with a doubling region allocator

[frankmcsherry]

What

Reworks RowArena from "one Vec<u8> allocation per push_bytes, all freed on clear" into a bump allocator over a stack of byte regions:

• push_bytes copies bytes into the active (last) region. When that region lacks spare capacity it allocates a new, larger region (doubling) rather than growing the current one — so a region that already holds data is never reallocated and references returned earlier stay valid. The outer Vec may reallocate as regions are added, but that only moves the Vec<u8> headers, not the heap buffers they own.
• push_bytes now accepts any B: Deref<Target = [u8]> (e.g. Vec<u8>, &[u8]). Existing Vec<u8> callers are unchanged; borrowed sources no longer need a throwaway allocation just to hand bytes over.
• clear retains only the single largest region (emptied) and drops the rest, right-sizing the arena. An arena reused across clear cycles — e.g. decoding arrangement rows one at a time — becomes allocation-free in steady state.
• reserve now reserves bytes in the active region (was: slots in the outer vector); with_capacity sizes the initial region. Neither has load-bearing callers (the reserve users in expr pass size hints).

Why

Follows up the ExtendDatums arena work (#37110) and the per-row arena hoist (#37113). Once the arena is the decode target for compressed arrangement rows (per-column codecs, #37111), a fresh allocation per value per row is the dominant cost; a reused doubling region removes it. The pattern mirrors columnation's region allocator.

The trade-off is the standard bump-allocator one: a single very large row leaves the arena holding a large region until the next clear. For the per-row-cleared decode/eval paths the high-water mark is just the largest single row, so it stays bounded.

Tests

Adds mz_ore::tests in repr::row covering:

• references from push_bytes remaining valid after later pushes force new regions,
• push_unary_row reading a row back from a non-zero region offset (confirms decoding is position/alignment independent),
• reuse across clear.

No behavior change for callers; no release note.

[antiguru]

Seems good!