feat: invoke registered page tools - #6

Resolves extension/src/probe.js, where this branch added invokeWebMcp
and cancelWebMcp while main rewrote probeWebMcp for type-checking.

Both sides are kept, with two changes the merge made worth doing:

The catalog normalization is now one shared function. Both copies did
the same work, and invokeWebMcp compares its normalized catalog against
the one the server recorded from probeWebMcp -- had the two ever
drifted apart, every invocation would have failed with stale_catalog.

executeTool() is documented as unspecified. It is not in the WebMCP IDL
or in webmcp-types; the upstream README still carries "TODO: Spec and
describe the modelContext.getTools() and modelContext.executeTool()
APIs", and webmachinelearning/webmcp#51 has been open since 2025-11-03.
getTools() was specced in #223, its sibling was not. The existing
feature-detect-and-report-unavailable behavior is unchanged and is what
section 21 of the design spec requires; it now passes through a single
narrow boundary so everything the spec does define stays type-checked.
Webby's own main-world global is declared in src/globals.d.ts rather
than loosened, keeping webmcp-types the only external contract asserted.