Write each rendered source line as one output operation

[Matei02355]

Colored Git logs produce many output writes for ANSI segments and sidebar cells, adding overhead when bat feeds a pager.

Render each source line into a reusable string and submit it as one output operation. Preserve byte-for-byte formatting, short-write handling, I/O errors and per-line streaming. Release buffers larger than 64 KiB after their line. Unbuffered mode still submits each available input fragment; it does not wait for the next source line.

Add regression coverage for write counts across character, word and unwrapped output, short writers and broken pipes. Include a standalone benchmark that compares identically built release binaries through a draining pager and verifies output hashes before alternating timed runs.

Validation: all 476 all-feature tests passed (7 ignored), including the three new regressions. Clippy with warnings denied, formatting, whitespace checks and the library-only build passed. Baseline and candidate both emitted a complete line and subsequent partial input before stdin EOF.

On this Linux host, eight alternating runs of an 80,000-line colored log through a cat pager improved from a 1.862 s median to 0.920 s (2.02x). One million numbered lines improved from 3.604 s to 2.503 s (1.44x). Rust highlighting was approximately unchanged (1.077 s versus 1.031 s). The unchanged plain-output path varied from 36.5 ms to 39.0 ms. All four output hashes matched. These measure formatting and pipe overhead, not terminal-emulator performance; no Windows speed claim is made.

Fixes #1147.