Add apx client verify generate-and-compile gate

[daniel-garcia]

Closes #40.

Problem

A spec can be valid OpenAPI 3 and pass apx lint + apx breaking, yet produce a client that does not compile. The release pipeline never generated or compiled a client, so a broken module could be tagged and published. Two constructs trigger it (both valid OAS3):

• redundant _limit/limit (+ _offset/offset) query params — oapi-codegen strips the leading underscore, so each pair collapses to the same Go field (Limit/Offset redeclared);
• a path parameter named url — the emitted url argument shadows the net/url import (url.Parse undefined).

Fix

Add apx client verify: generate a client and compile it, failing when any generated client does not build. This is the missing release gate.

• --warn-only (and release.verify_clients.warn_only) downgrades a failure to a warning (exit 0) for staged rollout; the default is to fail the gate.
• release.verify_clients.generators sets the matrix; default is go + typescript-angular.
• A generator whose toolchain is absent is skipped (not failed), so the gate covers Go always and TypeScript where Node is present.
• The Builder-or-npm build dispatch is centralized in internal/client and shared by generate --build, publish, and verify.

Tests

• Unit tests for client.Verify (pass / build-fail / generate-fail / skip / unknown generator / default matrix), using stub generators — no network.
• A regression fixture (testdata/goclient/collision.openapi.yaml) reproducing both patterns, plus network-gated E2E tests asserting the gate rejects it and accepts the good fixture.
• Docs (configuration.md, release-guardrails.md), CHANGELOG.md, and apx.example.yaml updated; schema/doc parity test passes.

Verified end-to-end on the fixture:

$ apx client verify --generator go --input testdata/goclient/collision.openapi.yaml
  go: FAILED — build: go build failed: exit status 1
  ./apxverifyclient.gen.go:21:2: Limit redeclared
  ./apxverifyclient.gen.go:23:2: Offset redeclared
  ./apxverifyclient.gen.go:238:24: url.Parse undefined (type string has no field or method Parse)
client verification failed: one or more generated clients did not build   # exit 1

$ apx client verify --generator go --warn-only --input testdata/goclient/collision.openapi.yaml
Client verification failed, but --warn-only is set — not failing the gate.  # exit 0