chore(entity-caching): benchmark suite and dev tooling (6/6)

[SkArchon]

Replaces #2947, which was auto-closed as merged when a bad force-push momentarily collapsed the stack branches onto one commit. Same content, same stack position.

Part 6 of 6 — split out from #2777. Stacked on the playground/studio PR.

What's included

• benchmark: k6 cache-demo scenarios, fixtures, response-equivalence verification, Redis/pprof capture scripts, suite runner + tests
• perf(router): no-op tracer provider when tracing has no realistic export destination (no graph token + only the default Cosmo Cloud exporter) — benchmark-motivated (~60% of allocations at 50 VUs were span work exporting to nowhere)
• root Makefile targets and .gitignore entries for the demo/benchmark stacks
• CLAUDE.md session notes and composition meta-docs (composition/CLAUDE.md, AGENTS.md, COMPOSITION_CONVENTIONS.md)

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.89%. Comparing base (4d87678) to head (1316bd8).

Files with missing lines	Patch %	Lines
router/pkg/trace/config.go	0.00%	15 Missing ⚠️
router/core/router.go	75.00%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@                              Coverage Diff                               @@
##           milinda/entity-caching-3-playground-studio    #2954      +/-   ##
==============================================================================
- Coverage                                       46.22%   44.89%   -1.34%     
==============================================================================
  Files                                            1121     1101      -20     
  Lines                                          158932   154457    -4475     
  Branches                                        11122    10123     -999     
==============================================================================
- Hits                                            73464    69341    -4123     
+ Misses                                          83515    83158     -357     
- Partials                                         1953     1958       +5     

Files with missing lines	Coverage Δ
router/core/router.go	66.24% <75.00%> (-0.45%)	⬇️
router/pkg/trace/config.go	55.93% <0.00%> (-19.07%)	⬇️

... and 46 files with indirect coverage changes

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[github-actions]

Router image scan failed

❌ Security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-96b34a7ad610c2b77f8b958a28fe1466768d89dc

Please check the security vulnerabilities found in the PR.

If you believe this is a false positive, please add the vulnerability to the .trivyignore file and re-run the scan.