[Feature] Bundle-size budget per subpath export (size-limit gate)

[pathosDev]

Motivation

"Batteries included, dependencies excluded" is the positioning (#447 §2) — a held size/tree-shaking promise is its proof. Nothing currently tracks the import cost of the package.

Proposal

• Track the tree-shaken import cost per subpath export (., ./testkit, ./devtools — the devtools path embeds the UI bundle, so it gets its own, larger budget) once [Feature] Add subpath exports to package.json #414 adds more subpaths
• size-limit (or a bun build --minify probe script) in package-health.yml; budgets committed; failure = a conscious, in-review budget bump
• Side benefit: [Feature] Built-in zero-dependency HTTP backend as default; Fastify becomes an optional peer #413 (fastify out of the runtime closure) should show up as a measured win, and any accidental heavy import shows up immediately

Acceptance sketch

• Budgets file + CI check with actionable output (delta per subpath)
• Budgets documented (README/docs mention the guarantee)
• Baseline recorded before/after [Feature] Built-in zero-dependency HTTP backend as default; Fastify becomes an optional peer #413 for the marketing number

Relates to #414, #413, #419.