PHLEGM v6 GUI compile / diagnostic follow-up

Observed compiler run: 2026-1002-1751-2580
- drawchr.lib completed PASS.
- the run stopped while compiling gui.lib.
- Build stderr was empty and the Compiler UI reported no readable exit-code sidecar.

Changes in this repair checkpoint:
- CHR File Tools retains independent Source BIN and Reference BIN tree/icon selection.
- GuiPopulateChrDirectory now takes an explicit selection-kind argument at every call site; no optional default was added to the established GUI routine.
- New CHR-selection branches are expanded into conservative multiline FreeBASIC conditionals.
- gui.lib no longer requires remap-state String label callbacks from the host; CHR REMAP STATUS remains the authoritative display of current file-tool selections/settings.
- Build-PHLEGM-v6.ps1 temporarily lowers ErrorActionPreference to Continue only while each native FBC command runs, captures stdout/stderr into the normal project compiler log, records LASTEXITCODE, then restores the prior preference.

The machine-facing behavior remains unchanged:
- CHR LOAD explicitly loads selected CHR media into the active machine-defined Memory / Program target.
- CHR SAVE explicitly pulls the current machine-defined CHR contents to a BIN.
- CHR REMAP remains file-to-file: two independently selected BIN inputs create a new third BIN without consulting or mutating active machine state.

Session behavior remains:
- named SESSION-SAVE / named SESSION-NEW confirm before overwrite of an existing named .session.phlegm.
- the blank-prefix .session.phlegm backup remains the automatic overwrite exception.
- SESSION-END confirms before removing an existing session target.
