refactor(stdlib): centralize string header readers

[proggeramlug]

Closes #8431

Summary

• centralize perry-stdlib StringHeader reads behind owned strict, lossy, and raw-byte accessors built on the runtime string byte chokepoint
• replace all 34 stdlib reader definitions while preserving each call site's prior UTF-8 behavior
• move the remaining Fastify, HTTP server, and net readers onto perry-ffi read_string/read_bytes
• retain owned returns so no GC-managed payload borrow escapes into allocating or async code

No version bump.

Validation

• cargo check -p perry-stdlib -p perry-ext-fastify -p perry-ext-http -p perry-ext-net
• cargo check -p perry-stdlib --tests
• cargo test -p perry-stdlib string_header_tests --lib (2 passed)
• Fastify reader/body tests passed; two unrelated GC relocation assertions failed in the full Fastify lib run
• cargo fmt --all
• git diff --check

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📥 Commits

Reviewing files that changed from the base of the PR and between 526e0b5 and 5768dce.

📒 Files selected for processing (42)

• changelog.d/8448-centralize-string-readers.md
• crates/perry-ext-fastify/src/context.rs
• crates/perry-ext-fastify/src/server.rs
• crates/perry-ext-http/src/server/types.rs
• crates/perry-ext-net/src/jsvalue.rs
• crates/perry-stdlib/src/argon2.rs
• crates/perry-stdlib/src/axios.rs
• crates/perry-stdlib/src/bcrypt.rs
• crates/perry-stdlib/src/cheerio.rs
• crates/perry-stdlib/src/commander.rs
• crates/perry-stdlib/src/common/mod.rs
• crates/perry-stdlib/src/container/mod.rs
• crates/perry-stdlib/src/container/types.rs
• crates/perry-stdlib/src/cron.rs
• crates/perry-stdlib/src/crypto/util.rs
• crates/perry-stdlib/src/dayjs.rs
• crates/perry-stdlib/src/decimal.rs
• crates/perry-stdlib/src/domain.rs
• crates/perry-stdlib/src/dotenv.rs
• crates/perry-stdlib/src/events.rs
• crates/perry-stdlib/src/fetch/mod.rs
• crates/perry-stdlib/src/framework/multipart.rs
• crates/perry-stdlib/src/framework/request.rs
• crates/perry-stdlib/src/framework/response.rs
• crates/perry-stdlib/src/framework/server.rs
• crates/perry-stdlib/src/ioredis.rs
• crates/perry-stdlib/src/jsonwebtoken.rs
• crates/perry-stdlib/src/lodash.rs
• crates/perry-stdlib/src/moment.rs
• crates/perry-stdlib/src/mongodb.rs
• crates/perry-stdlib/src/nanoid.rs
• crates/perry-stdlib/src/net/mod.rs
• crates/perry-stdlib/src/ratelimit.rs
• crates/perry-stdlib/src/readline/mod.rs
• crates/perry-stdlib/src/sharp.rs
• crates/perry-stdlib/src/slugify.rs
• crates/perry-stdlib/src/sqlite/options.rs
• crates/perry-stdlib/src/tls.rs
• crates/perry-stdlib/src/uuid.rs
• crates/perry-stdlib/src/validator.rs
• crates/perry-stdlib/src/worker_threads.rs
• crates/perry-stdlib/src/ws.rs

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[proggeramlug]

Validated as part of an 11-PR batch (#8439, #8440, #8441, #8442, #8443, #8444, #8446,
#8448, #8450, #8453, #8454) stacked on main and built once, then merged individually.

• 19/19 sweep corpus byte-exact against the Node oracle
• perry-runtime --lib 2601 · perry --bin perry 1007 · perry-codegen --lib 1113
• scripts/run_lint_gates.sh — all 50 gates
• A re-entrancy probe (user JS re-entering via toString/valueOf/replacer/comparator
  during normalize, repeat, regex lastIndex coercion, replace, JSON.stringify,
  sort, and punycode host conversion) matches Node exactly, including under
  PERRY_GC_FORCE_EVACUATE=1 PERRY_GC_VERIFY_EVACUATION=1.

One thing stated plainly: these are hardening, not demonstrated repairs

I could not make the underlying bugs reproduce. My probe passes on unmodified main, and so
do all four of the fixtures this series ships
(test_issue_8426_normalize_reentrant, test_issue_8428_exec_lastindex_reentrant,
test_gap_gc_string_copy_source_rooting, test_gap_gc_string_repeat_reentrant_count) —
including under PERRY_GC_FORCE_EVACUATE=1, PERRY_GC_VERIFY_EVACUATION=1, and
PERRY_GC_PROTECT_FROMSPACE=1 at depth 800.

That is consistent with the string audit having found these windows by reading rather than by
reproducing, and with this bug class being invisible at collection time. The changes are
still worth landing — an unrooted borrow across user JS is a real latent hazard. But the
merge rests on "correct by construction and regression-free", not on "fixes an observed
failure", and the fixtures should be understood as no-regression guards rather than
reproducers.