Compile-time AST lint pass detecting common string performance anti-patterns:
- PERF001: string += in loop (O(n^2))
- PERF002: character_at with loop variable index (O(n) per call)
- PERF003: character_at anywhere (informational)
- PERF004: string interpolation reassignment in loop (O(n^2))
- PERF005: length(string) in while condition (strlen each iteration)

Uses counter-based detection via visitor state to catch loop variables
nested in expressions (e.g. character_at(s, i+1)). Includes proper
variable scope tracking, closure isolation, and deprecation of character_at.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…ucture

Replace parsing-based warning detection in standalone tool with direct
perf_lint(prog, false) call. Make test functions optimization-safe by
adding parameters and return values. Remove makeFunctionDeprecated from
C++ (character_at deprecation now handled by PERF003 lint rule).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Add O(1) first_character() for both string and das_string (throws on empty)
- Add with_das_string() to create temporary das_string for block scope
- Optimize character_at() to scan only to index instead of full strlen
- Replace character_at usage in aot_cpp.das with first_character, slice, and fixed_array indexing
- Add perf_lint.rst to language toctree, fix Sphinx warnings
- Add handmade docs for new functions and macro_performance_warning
- Tests: 5 subtests covering both overloads, empty-throws, and bounds checks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Rename perf004_* fields to descriptive string_builder_* names
- Replace print() with to_log(LOG_WARNING) in perf_lint.das
- Flatten nested ifs with postfix conditional returns
- Early-return guard in visitExprCall using postfix syntax
- Remove spaces before ++/-- operators (project convention)
- Move visitor adapter from class field to local variable
- Shorten test docs section in perf_lint.rst
- Replace if(true) with bare scope block in main.das
- Remove unnecessary threadlock_context and unsafe wrapper

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Add performance lint module, first_character/with_das_string builtins, optimize character_at

- Fix PERF001 (string += in loop) in constant_expression, coverage,
  interfaces, rst (5 functions converted to build_string)
- Fix PERF003 (character_at(s,0)) in json_boost, regex, rst,
  strings_boost by replacing with first_character
- Fix PERF002 (character_at in loop) in regex expand_replacement
  and strings_boost is_null_or_whitespace using peek_data
- Improve perf_lint: detect character_at(s, 0) and suggest
  first_character in PERF003 message
- Fix perf_lint: PERF003 now fires inside closures (was incorrectly
  skipped by in_closure guard)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Fix PERF001 warning in split_sptn -- character-by-character string
building replaced with strings_boost split().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- 27_testing: replace character_at loop with peek_data in is_palindrome
- 42_testing_tools: replace string += loop with build_string in reverse_string
- call_macro_mod: cache length(format) outside while condition (PERF005)
- reader_macro_mod: replace string += loop with build_string
- typeinfo_macro_mod: replace string += loop with build_string
- 07_sse_and_streaming: replace character_at loop with peek_data in SSE parser
- Update 4 RST files with matching code changes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Fix perf_lint warnings across daslib, modules, tutorials