refactor: migrate from zap to log/slog

[dmke]

Replace go.uber.org/zap logging with Go's standard log/slog package throughout the codebase. Introduces thin wrapper module (xlog) that provides a simplified interface around slog with:

• Logger interface with Debug/Info/Warn/Error/Fatal methods
• Functional options pattern for configuration
• Support for both text (via slogor) and JSON handlers
• Convenience attribute generators (String, Int, Error, etc.)
• Test utilities (MockClock, Discard)

Notable Changes:

• add xlog/ module with wrapper implementation and tests
• migrate all packages
• update logger setup in cmd/texd/ to use xlog
• adapt all test files to use xlog.NewDiscard() and test helpers

Fixes: #1
Closes: #108

[codecov]

Codecov Report

❌ Patch coverage is 85.84475% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.64%. Comparing base (0779dbc) to head (7b2d7c0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
service/renderer.go	43.75%	9 Missing ⚠️
cmd/texd/setup.go	66.66%	5 Missing ⚠️
cmd/texd/signals.go	0.00%	4 Missing ⚠️
service/service.go	57.14%	3 Missing ⚠️
xlog/discard.go	75.00%	3 Missing ⚠️
xlog/xlog.go	94.54%	3 Missing ⚠️
refstore/nop/nop_store.go	0.00%	2 Missing ⚠️
cmd/texd/main.go	0.00%	1 Missing ⚠️
service/middleware/logging.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   78.47%   79.64%   +1.16%     
==========================================
  Files          39       43       +4     
  Lines        1719     1837     +118     
==========================================
+ Hits         1349     1463     +114     
- Misses        300      305       +5     
+ Partials       70       69       -1     

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