Move seven pure-string path functions (_ChangeFileExt, _ExtractFileName,
_ExtractFilePath, _ExtractFileDir, _ExtractFileExt,
_IncludeTrailingPathDelimiter, _ExcludeTrailingPathDelimiter) from
blaise_io.c to blaise_str.pas, reducing C code by ~130 lines while
keeping all OS-level functions (rmdir, rename, chdir) in C.
Separate always-linked runtime code (system.pas, ARC, strings, platform
abstraction) from opt-in standard library units (sysutils, classes, math,
dateutils, generics, etc.). This mirrors the Go runtime/ vs stdlib, Rust
core vs std, and Swift SwiftRuntime vs SwiftCore patterns.
- runtime/ contains code linked into every binary + C shims
- stdlib/ contains units imported explicitly via uses clause
- Single platform abstraction layer (rtl.platform.*) in runtime/,
shared by both runtime/ and stdlib/ units
- bcl.testing renamed to blaise.testing across all 95+ test files
- Updated PasBuild project.xml, fixpoint.sh, README, unit search paths
- All 1931 tests pass, fixpoint OK (stage-3/stage-4)
