Gnome: bidi-isolate inline code in RTL MDX views

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Fixes #117

Problem

In RTL locales, neutral characters (quotes, dots, colons) next to LTR runs could resolve to the wrong side. The root cause for the asymmetric case reported in #117: when punctuation sits between two LTR runs — typically an inline-code mnemonic and a following English word — the Unicode bidi algorithm joins everything into one left-to-right run.

Real example from the Hebrew tutorial (<tt>BNE</tt>: "Branch on not equal" inside a Hebrew sentence): the colon and the opening quote stuck to the LTR run while the closing quote correctly resolved right-to-left — exactly the "quote before but not after LTR letters" asymmetry described in the issue.

Note: the simpler cases from the issue (punctuation between an LTR run and Hebrew text, or at line end) were already fixed by the RTL paragraph direction from #121; this PR fixes the remaining run-joining case.

Fix

In MdxView.applyRtlBaseDirection, additionally wrap the content of <tt> spans in FIRST STRONG ISOLATE (U+2066–U+2069 family: FSI U+2068 / PDI U+2069) characters. Isolated runs are treated as neutral by the surrounding text, so all adjacent punctuation now resolves uniformly against the RTL paragraph direction, while the bidi algorithm still renders the isolated code itself left-to-right. FSI (first-strong) is used instead of LRI so the span direction always derives from its content. The transform is idempotent.

Verification

GJS integration test loading the real generated tutorial.ui with the real Hebrew translations and Gtk.TextDirection.RTL (mirroring the #121 methodology):

	without fix	with fix
Labels with Hebrew text	144	144
Hebrew labels with <tt> spans	84	84
Spans bidi-isolated	0	84
Labels with broken Pango markup	0	0
Hebrew labels rendered LTR (#116 regression check)	0	0
BNE paragraph: colon RTL-positioned	❌	✅
BNE paragraph: quotes hug the quoted phrase symmetrically	❌	✅

Glyph positions verified via PangoLayout.index_to_pos on the rendered labels.

Also passing: yarn format, yarn check:format, yarn build, app-gnome type check (blueprint validation included).

Notes

• Code blocks (SourceView) keep their intentional LTR layout, unchanged.
• The Android app renders the same MDX content with TextView/HTML and likely needs an equivalent fix — separate follow-up, same as noted in Gnome: force RTL base direction in MDX views #121.