refactor(deps): declare dependency features in member crates

[wan9chi]

Motivation

Features declared on [workspace.dependencies] entries are forced on every member that uses the dependency — a member can add features but never subtract them — so the workspace manifest quietly over-featured most crates and hid which crate actually needs what. Every features = [...] list moves from the workspace manifest to the member crates that use it; default-features = false stays at the workspace level, where it is the only place it can take effect.

Moving them exposed two kinds of rot:

• Dead features: bumpalo/collections was declared workspace-wide for one brace-grouped import in fspy's arena (now declared there); nix/dir was used only by vt's fingerprinting; nix/signal only by pty_terminal's tests; xxhash-rust/const_xxh3 had no users at all and is gone.
• Masked under-declarations: fspy_ipc_str uses TransparentWrapperAlloc (extern_crate_alloc) but only compiled because the workspace forced the feature; it now declares it.

Every affected member was checked individually (cargo check -p … --all-targets) on the host plus the Linux and Windows targets, so no member relies on feature unification from a sibling.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.13%  [-12.11% .. +14.72%]  overhead  +285.49%
dynamic/access             change  -0.37%  [-15.47% ..  +1.93%]  overhead    +7.76%
dynamic/access-relative    change  +0.45%  [-13.26% ..  +4.44%]  overhead   +48.08%
dynamic/access-contended   change  +1.68%  [ -3.85% .. +15.64%]  overhead   +10.99%
static/launch              change  +1.17%  [ -9.69% .. +12.30%]  overhead  +637.41%
static/access              change  -0.02%  [ -8.00% ..  +9.40%]  overhead  +829.07%
static/access-relative     change  +1.62%  [ -6.23% ..  +8.42%]  overhead +1171.13%
static/access-contended    change  -2.30%  [-13.51% ..  +6.40%]  overhead +1993.69%

macos

dynamic/launch             change  -0.96%  [ -4.29% ..  +3.85%]  overhead  +215.70%
dynamic/access             change  +0.36%  [ -2.51% ..  +3.77%]  overhead    +4.84%
dynamic/access-relative    change  +0.00%  [ -2.28% ..  +2.70%]  overhead  +259.03%
dynamic/access-contended   change  +0.28%  [ -7.71% ..  +6.09%]  overhead    +4.19%

windows

dynamic/launch             change  +0.08%  [ -5.52% ..  +4.80%]  overhead   +25.74%
dynamic/access             change  -0.19%  [ -1.26% ..  +1.29%]  overhead    +1.33%
dynamic/access-relative    change  +0.36%  [ -0.91% ..  +1.27%]  overhead    +1.83%
dynamic/access-contended   change  -0.45%  [ -2.56% ..  +1.91%]  overhead    +2.12%