Release v0.3.3

[iainmcgin]

Lockstep bump of connectrpc, connectrpc-build, and connectrpc-codegen to 0.3.3.

Headline

• connectrpc-build: skip rerun-if-changed for .files() in Precompiled mode #56 — connectrpc-build no longer emits cargo:rerun-if-changed
  directives for .files() when Precompiled input mode is used.
  Consumers that feed a precompiled FileDescriptorSet were getting
  spurious rebuild triggers on paths that don't apply in that mode.

Also included since v0.3.2

• Pin MSRV to Rust 1.88 and verify in CI #44 — workspace declares rust-version = "1.88" and CI verifies
  it. The code already required 1.88 (let-chains) in v0.3.2; this
  commit documents the requirement.
• Add streaming-tour and middleware examples #46 — new examples/streaming-tour and examples/middleware
  crates. Not published.
• Add user guide and adoption-helpful README signposts #48 — user guide at docs/guide.md, linked from README. Not
  published.

No public API changes in connectrpc. connectrpc-codegen only gained
a one-line intra-doc-link tweak in a generated doc comment (#46).

Checklist

• All three crate versions bumped to 0.3.3 (lockstep).
• Inter-crate dependencies remain version = "0.3" (compatible).
• CHANGELOG.md updated with the 0.3.3 entry and PR refs.
• CI green.
• Tag v0.3.3 and run the publish workflow after merge.