perf(octicons): cache embedded data URIs on demand

[dexhunter]

Summary

Cache successful embedded Octicon data-URI reads on first use so repeated stateless server and tool registration no longer rereads and base64-encodes the same PNGs. Missing lookups remain uncached, and every Icons call still returns a fresh mutable slice.

Why

Fixes#2850

Repeated all-tool registration currently allocates a median 193,085 B/op and 527 allocs/op for the 31 required icons after warmup. The same process may rebuild this inventory for multiple stateless HTTP requests.

What changed

• Add a race-safe, on-demand cache for successful (name, theme)DataURI reads.
• Preserve lookup for every embedded PNG, including files not listed in required_icons.txt.
• Add missing-lookup, all-embedded-file, mutation-isolation, and concurrent-first-use tests.
• Add reproducible cold and warm benchmarks for one-icon and default inventories.

Performance

Inventory	State	Main B/op	This PR B/op	Main allocs/op	This PR allocs/op
31 icons	first registration	240,464	247,080	534	538
31 icons	warm registration	193,085	4,464	527	31
1 icon	first registration	53,592	60,208	23	27
1 icon	warm registration	4,160	144	17	1

For the default inventory, warm allocation bytes fall by 97.69% and allocation count falls by 94.12%. The disclosed cold cost is +6,616 B and +4 allocations while the cache is populated. No icon is read or encoded in init().

Supplementary autoresearch: 20-step public dashboard.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• Not applicable; this does not change tool schemas or behavior.

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Only embedded PNG data is cached. Missing lookups are not retained. With all current variants used, retained data-URI payload is 46,840 bytes plus bounded map overhead.

Tool renaming

• I am renaming tools as part of this PR
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test
• go test -race ./...
• Strict cold/warm evaluator with concurrent-first-use race coverage
• git diff --check

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR optimizes the pkg/octicons package by adding an on-demand, race-safe cache for successfully read embedded Octicon PNG data URIs, reducing repeated base64 encoding work during repeated tool/server registrations while preserving existing behavior for misses and returning fresh icon slices.

Changes:

• Introduced an RWMutex-protected (name, theme) -> data URI cache for successful embedded icon reads, leaving missing lookups uncached.
• Updated DataURI to use the cache while keeping the underlying embedded read logic in a dedicated helper.
• Added tests for all embedded icons, cache semantics (misses not cached), mutation isolation, and concurrent first-use; added cold/warm benchmarks.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
pkg/octicons/octicons.go	Adds a race-safe on-demand cache and routes DataURI through it.
pkg/octicons/octicons_test.go	Adds test coverage for cache behavior, embedded inventory coverage, concurrency, and slice mutation isolation.
pkg/octicons/octicons_benchmark_test.go	Adds benchmarks for cold/warm cache behavior and icon registration allocation profiles.

[SamMorrowDrums]

Superseded by #2895, which builds on this performance improvement by moving the base64 work to icon generation time. Thanks @dexhunter for identifying the repeated encoding overhead and proposing the caching improvement.