docs: add Open Collective sponsorship badges

[kolkov]

Summary

• Added Financial Contributors badge in header badge row
• Added Sponsors/Backers section before License with auto-updating SVG avatars from Open Collective
• Added "Sponsor on Open Collective" button

Collective page: https://opencollective.com/coregx

Test plan

• Verify badge renders correctly on GitHub
• Verify Sponsors/Backers SVG loads (may show empty until first sponsor)
• Verify "Sponsor" button links to correct collective

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[github-actions]

Benchmark Comparison

Comparing main → PR #159

Summary: geomean 82.47n 82.39n -0.10%

⚠️ Potential regressions detected:

geomean                               ³                +0.00%               ³
geomean                               ³                +0.00%               ³
geomean                         ³                +0.00%               ³
geomean                         ³                +0.00%               ³
DNA_VsStdlib/stdlib/dna_8-4                             61.96m ± ∞ ¹    77.08m ± ∞ ¹    +24.42% (p=0.032 n=5)
LangArenaLogParser/ips-4                                47.83µ ± ∞ ¹    48.45µ ± ∞ ¹     +1.28% (p=0.008 n=5)
LangArenaLogParserStdlib/ips-4                          44.98m ± ∞ ¹    54.53m ± ∞ ¹    +21.21% (p=0.008 n=5)
LangArenaLogParserStdlib/api_calls-4                    3.961m ± ∞ ¹    4.167m ± ∞ ¹     +5.22% (p=0.032 n=5)
LangArenaLogParserStdlib/methods-4                      71.08m ± ∞ ¹    75.93m ± ∞ ¹     +6.82% (p=0.008 n=5)
BranchDispatch_Stdlib/Digits-4                          131.9n ± ∞ ¹    133.0n ± ∞ ¹     +0.83% (p=0.008 n=5)

Full results available in workflow artifacts. CI runners have ~10-20% variance.
For accurate benchmarks, run locally: ./scripts/bench.sh --compare