Move TypeScript bindings to bindings/ts

[escapedcat]

Follow-up to #103, per post-merge feedback: keep the top-level bindings/ directory language-neutral so bindings for other languages (Kotlin, Swift, ...) can live alongside the TypeScript ones later.

What this does

• Moves the committed bindings from bindings/ to bindings/ts/
• Sets the default export directory via [env] in .cargo/config.toml (relative = true, so it resolves against the repo root regardless of where cargo runs). A plain cargo test keeps regenerating the committed files in place
• devtools export-ts-types [dir] is unaffected: it sets TS_RS_EXPORT_DIR explicitly, and cargo's [env] default only applies when the variable is unset — verified both paths
• The CI freshness check keeps watching all of bindings/, so future languages inherit it for free
• README / AGENTS.md paths updated

The frontend branch (teambtcmap/btcmap.org#1243, unmerged) already points its pnpm types:api at bindings/ts — it fetches from master, so it needs this PR to land first.

Binary-size measurement for the perf question raised after #103 is running; numbers will follow as a comment.

🤖 Generated with Claude Code

[escapedcat]

Perf numbers for the question raised after #103, measured on the merge commit vs its first parent (identical toolchain, cargo build --release with the repo's native-CPU flags):

build	target/release/btcmap-api
08b8c18^1 (pre ts-rs)	46,409,136 bytes
08b8c18 (with ts-rs)	46,408,416 bytes

Δ = −720 bytes — link-layout noise. The generated TS impls are only referenced by test code, so the linker dead-strips them from the release binary; runtime has no ts-rs code path at all. The only real cost is compile time for two extra crates (ts-rs + its proc-macro) on clean builds.