Match downloaded ZLS version to the installed Zig toolchain

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Summary

Fixes #36 and #38.

The extension always fetched the latest zigtools/zls GitHub release regardless of which Zig version was actually installed. ZLS refuses to run against a mismatched Zig version, so once zls moved ahead of a user's Zig install (e.g. Zig 0.16 while zls was still 0.15), the extension broke with:

ZLS '0.15.0' does not support Zig '0.16.0'. A ZLS '0.16' release should be used instead.

This also affected dev builds, as reported in #38.

Changes

• detect_zig_version resolves zig via the worktree's PATH (respecting version managers like mise/asdf that alter the shell env) and runs zig version to get the installed version.
• matching_zls_release queries zigtools' select-version API to fetch the ZLS build that actually matches the installed Zig version, using the API-provided tarball URL directly (handles both the current {arch}-{os} naming and legacy {os}-{arch} naming from older ZLS releases).
• Falls back to the previous "latest GitHub release" behavior only when no zig binary can be found on PATH at all.
• extension.toml gains a process:exec capability for zig version, required by Zed's WASM sandbox.
• ZigExtension's binary-path cache is now keyed by detected Zig version (instead of a single global cache), since different worktrees may need different ZLS builds concurrently. Added a bounded cleanup pass that removes zls-* directories no longer referenced by the current session's cache, so this doesn't grow disk usage unboundedly.
• Added unit tests for the new pure logic (version-string parsing, URL construction, response parsing including the API's error-shaped payloads).

Test plan

• cargo test — 12/12 unit tests pass
• cargo clippy --target wasm32-wasip2 --release — no warnings
• cargo fmt --check — clean
• cargo build --release --target wasm32-wasip2 — builds successfully
• Manually verified against a real Zig 0.16.0 install: zig version → 0.16.0, and https://releases.zigtools.org/v1/zls/select-version?zig_version=0.16.0&compatibility=only-runtime returns a matching ZLS 0.16.0 release with a working tarball URL
• Not tested inside the actual Zed editor as a dev extension (no GUI available in the environment this was developed in) — would appreciate a test run from a maintainer or reviewer with a Zed GUI available

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