build(xtask): remove temporary tinyvec fixture pin

[mdorman]

Problem

rendered-html-compiler-boundary deliberately creates a lockless external crate to test common as a downstream dependency. On 2026-09-03, the crates.io index began resolving its transitive tinyvec = "1" requirement to 1.13.0. That release fails under alloc without std because TinyVec::with_initial_len invokes an unqualified vec! macro.

The compiler fixture now constrains tinyvec to 1.11.0 so the unrelated upstream regression does not block Jaunder's public/private API boundary gate.

Upstream:

• vec macro import error with 1.13.0 Lokathor/tinyvec#225
• Fix no std with alloc Lokathor/tinyvec#226

Acceptance

• A released tinyvec version containing the upstream fix is available.
• Removing the fixture's direct exact tinyvec dependency leaves the lockless external fixture compiling with common default features disabled.
• The positive, raw-constructor privacy, and test-support confinement compiler contracts still pass.
• No production dependency or feature is added to compensate for the fixture-only workaround.