Check exact-empty sections across all libraries

[richlander]

Exact-empty validation now evaluates the whole multi-assembly document instead of only its first inspection. A selected section renders when any assembly has rows, and fails only when every assembly is empty.

Markdown output now composes each assembly with its own effective section filter under the package's Libraries heading. This preserves the report hierarchy while ensuring a populated later assembly renders without leaking unrequested sections. Row-oriented formats retain their existing per-assembly rendering.

The same document-level decision governs empty-section notes: a populated later assembly no longer appears alongside a false note that the selected section has no data. Inspection failures remain correlated per assembly, so a failure in one assembly is still visible even when another assembly renders the requested section. Duplicate identical failures are reported once.

A deterministic local package fixture puts an empty assembly first and the test assembly with async methods second. It pins both sides:

• -S "Async Methods" --tsv and --markdown exit 0 and emit the later assembly's rows.
• Markdown retains the package/library hierarchy and does not leak Library Info or Symbols.
• -S Resources --tsv still exits 1 with empty stdout when every assembly is empty.
• The first assembly's zero count is asserted to keep the regression fixture non-vacuous.
• A synthetic mixed-inspection test pins failure visibility when a later assembly has data.

Compatibility boundary: single-assembly behavior is unchanged. Singleton --count, wildcard provenance (#3886), and multi-assembly reference trees (#3885) are separate paths and remain out of scope.

Validation:

• Release solution build succeeds.
• Full CLI and inspection-workspace query suites pass.
• Real package canary: Microsoft.NETCore.App.Runtime.linux-x64@10.0.0 --tfm all -S "Async Methods" --tsv exits 0 with 269 lines and no stderr (regressed head exited 1 with empty stdout).

Closes #3884.

[richlander]

Closing as superseded. #3901 merged the document-wide exact-empty fix for #3884, and #3945 subsequently added the multi-inspection Markdown framing, aggregate count behavior, wildcard boundary, and per-assembly failure diagnostics while defining the current output-shape contract. This branch no longer carries a unique behavioral change worth resolving against current main.