Add checked parsing, BtechTextBuilder, and modernize string/file/event handling

[gtaylor]

Motivation

• Replace ad-hoc, unsafe numeric parsing and legacy file shelling with robust checked parsers and direct file I/O to avoid overflow, trailing-text, and shell-exec issues.
• Provide a small bounded text builder API to simplify safe incremental formatted output and avoid repeated manual buffer manipulation.
• Normalize string utilities and event failure handling so repair events can be persisted and restored deterministically.

Description

• Added checked parsing helpers in mux/support/stringutil.{c,h}: parse_int_checked, parse_long_checked, parse_float_checked, plus supporting helpers and improved string_* APIs and matches_exit_from_list implementation; exported new prototypes in the header.
• Introduced BtechTextBuilder with btech_text_builder.{h,c} and replaced many ad-hoc strcat/snprintf manipulations with builder calls in multiple UI files (mech_status_summary.c, mech_status_weapons_format.c, etc.).
• Replaced legacy compressed-file helpers (my_open_file/my_close_file/template_file.c/mechfile_api.h) with plain fopen/fclose usage and removed the generated mech file API.
• Added a persistent marker for failed repair events: renamed/added mech_event_failure_marker and replaced prior very_fake_func usages and prototypes across event/repair persistence and dispatch code.
• Updated many callsites to use parse_*_checked instead of atoi/atof/strtol, added #include "mux/support/stringutil.h" where needed, and adjusted function names for stagger modifiers to clearer identifiers.
• Build/test integration: added core/btech_text_builder.c to BTECH_SOURCES, added tests/btech_safe_helpers.c and a btech_safe_helpers_test executable and test entry in CMakeLists.txt (with -ffunction-sections/--gc-sections for test linking).

Testing

• Added and executed the unit test btech_safe_helpers (registered in CTest as btech_safe_helpers) which exercises checked parsing and BtechTextBuilder; the test passed.
• Performed a CMake build of the modified targets including the new test executable; build completed and the new test was run via ctest -R btech_safe_helpers successfully.

---

Codex Task