perf: implement letter based skip index for token blocks

[cheb0]

Description

Using 30 bits per table entry: 26 for eng letters, 2 for utt-8 ru letter bytes, 1 bit for digits, 1 bit for the rest.

Largely depends on data store in a database, but some real world installations can get these numbers

Query	env	cold, ms		hot, ms		cold (branch), ms		hot (branch), ms		cold diff	hot diff
message:*content*	prod	211.61	±4.78	35.31	±0.18	155.99	±2.62	15.21	±0.19	-26.3%	-56.9%
message:*аннуляци*	prod	154.65	±5.02	16.48	±0.17	81.47	±2.67	3.09	±0.27	-47.3%	-81.3%
message:*applicationexception*	prod	131.17	±6.44	14.3	±0.11	85.19	±3.52	5.93	±0.27	-35.1%	-58.5%
message:*3645320973434*	prod	133.99	±5.37	19.47	±0.16	139.56	±8.21	19.27	±0.16	4.2%	-1%

---

• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
Click on Show table button to see full list of degraded benchmarks.

Show table

Name	Previous	Current	Ratio	Verdict
Not/size=10000-4	25381c	45af0f
	4.51 ns/op	5.40 ns/op	1.20	🔴
NotEmpty/size=1000-4	25381c	45af0f
	4.53 ns/op	5.04 ns/op	1.11	🔴
Or/size=1000000-4	25381c	45af0f
	4.27 ns/op	4.75 ns/op	1.11	🔴

[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
Click on Show table button to see full list of degraded benchmarks.

Show table

Name	Previous	Current	Ratio	Verdict
BucketClean-4	25381c	f25e66
	40994.00 ns/op	45644.00 ns/op	1.11	🔴
MutexListAppend-4	25381c	f25e66
	199.21 MB/s	173.31 MB/s	0.87	🔴
	80797744.00 ns/op	92317994.00 ns/op	1.14	🔴

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.58824% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.59%. Comparing base (25381c2) to head (7409cdc).

Files with missing lines	Patch %	Lines
util/letters_bitset.go	87.87%	4 Missing ⚠️
frac/sealed/token/table_loader.go	70.00%	2 Missing and 1 partial ⚠️
cmd/index_analyzer/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
+ Coverage   70.51%   70.59%   +0.08%     
==========================================
  Files         232      233       +1     
  Lines       18268    18339      +71     
==========================================
+ Hits        12881    12946      +65     
- Misses       4426     4432       +6     
  Partials      961      961              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.