Accept ASCII in -Validate wherever BOM-less UTF-8 is allowed

[amrali-eg]

Problem

After #132, conversion to UTF-8 without a BOM leaves ASCII files unchanged ("ASCII is already valid UTF-8 without a BOM"). -Validate still compared labels, so the same pure-ASCII file failed. Reproduced with the master build:

-Validate list	Before
utf-8	Invalid / CharsetNotAllowed, exit 2 under -FailOnChanges
utf-8,utf-8-bom	same
utf-8-bom	same

A pipeline that converted with -Target utf-8 and then validated with -Validate "utf-8,utf-8-bom" (the example in CLI.md) failed every file whose text was plain English. It also contradicted the plan guarantee text "ASCII already counts as BOM-less UTF-8".

Change

• ConversionPolicy exposes the rule it already applies (IsAsciiAlreadyUtf8) and its explanation (AsciiAlreadyUtf8Reason). Conversion and -Validate now both use it, so they accept the same files.
• -Validate accepts a detected ASCII file when the list allows BOM-less utf-8, and explains the pass with "ASCII is already valid UTF-8 without a BOM." A list allowing only utf-8-bom still rejects it, because an ASCII file has no BOM.
• Full-file validation is unchanged: the whole file is still validated as ASCII, so a non-ASCII byte past the 64 KiB detection sample is reported Invalid / StrictValidationFailed, as conversion reports it.
• CLI.md notes the rule in the -Validate row; backlog entry BL-38.

No conversion decision changes, so the semantics version stays at 8.

Tests

In AsciiAlreadyUtf8Tests:

• A theory over utf-8, utf-8,utf-8-bom, us-ascii and utf-8-bom, asserting result, reason code, explanation and the -FailOnChanges exit code.
• A file with ASCII in the detection sample and a UTF-8 é after it: Invalid / StrictValidationFailed, exit 2.

Mutation checks (each restored byte-for-byte afterwards)

Mutation	Tests failing
The UTF-8 rule disabled in -Validate	3
Accepted ASCII skips full-file validation	2 (including the existing ConvertAndValidateNowAgreeAboutTheSameFile)
utf-8-bom also accepts ASCII	1

Verification

• Rebased onto master after Refuse a contradicting source choice even when it is the target #135; the backlog conflict was resolved by keeping both entries (BL-37, BL-38).
• Release build, 0 warnings; 967/967 tests pass.
• docs/Test-DefectBacklog.ps1 passes.
• GUI smoke not run locally for this PR; CI runs gui-smoke. The full smoke run and the four-corpus audit will be on the final committed release candidate.

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