fix(posting_guard): decode text spelled in invisible characters

clean_text removed zero-width, bidi, soft-hyphen and BOM characters,
but Unicode tag characters (U+E0000-E007F, which shadow ASCII one for
one) and runs of variation selectors (one byte each) went into the
cleaned text untouched. An instruction spelled in them renders as
nothing on the page or in the terminal and still reads as text to the
model, so the gate passed it as clean.

Both are now removed and counted. What a tag run or a selector run
spells is decoded into a new invisible_text finding, scanned like
hidden content (so instruction_like and hidden_urls see it), and
raises the verdict's warning. Emoji subdivision flags and a lone
variation selector are counted, not flagged. /apply's list of findings
to show gains the new one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -536,6 +536,19 @@ per-file diff commands.
   size) still needs a renderer and is not caught; the trust-boundary rule in
   `09-web-research.md` remains the defence there.
 
+- **`posting_guard.py` decodes text spelled in invisible characters** (`tools/posting_guard.py`,
+  `tests/test_posting_guard.py`, `/apply` Step 1 item 0) - #702 removes and counts Unicode tag
+  characters (U+E0000-E007F, which shadow ASCII one for one), but what they spelled was thrown
+  away, so an instruction written in them raised only a character count, never a quoted line;
+  and runs of variation selectors (one byte each) passed into the cleaned text untouched. An
+  instruction spelled either way renders as nothing on the page and in the terminal, yet reads
+  as text to the model. What a tag run or a selector run spells is now decoded into a new
+  `invisible_text` finding, scanned like hidden content (so `instruction_like` and
+  `hidden_urls` see it), and raises the verdict's ⚠ (`N strings spelled in invisible
+  characters`) next to #702's tag count. Variation selectors are removed and counted under
+  `invisible_chars.variation`; a lone one, such as the VS16 in ❤️, is counted as encoding noise
+  and never decoded. The three UK subdivision flag emoji are kept and not counted, as before.
+
 ## [1.7.3] - 2026-10-19
 
 ### Added