fix(buffer): align both write entry points on encoding tags

[proggeramlug]

Fixes #10645.

js_buffer_write handled hex and base64 but fell back to raw UTF-8 for utf16le/ucs2, base64url, latin1, and ascii tags. Both buffer write entry points now use the existing shared string-to-encoding helper, so their tag tables cannot drift independently.

A runtime unit test writes through both entry points with tags 0–6 and checks the byte counts, encoded bytes, and untouched surrounding bytes.

Verification: RUST_TEST_THREADS=1 cargo test -p perry-runtime --lib both_write_entry_points_decode_every_encoding_tag -- --test-threads=1, rustfmt --check --edition 2021 crates/perry-runtime/src/buffer/copy_write.rs, and git diff --check.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between c1569e2 and 36c3070.

📒 Files selected for processing (2)

• changelog.d/10987-buffer-write-encodings.md
• crates/perry-runtime/src/buffer/copy_write.rs

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[proggeramlug]

Landed on main in merge train 257 (#11039, v0.5.1640), main 990b3eeada.

Carried at head 36c3070cae. CI on the train head passed every job except the known public-baseline lint step: all 6 gap shards, cargo-test, e2e-scoped, gc-stress, check, warnings and security-audit green.

This train was split by blast radius after an earlier 35-PR assembly hit five gap regressions: it carries only PRs touching no lowering path. Trains rebase-merge, so commits get new SHAs and GitHub cannot mark this merged. Closed as landed.