core: build Welcome text from Core's Intro strings

[BenWestgate]

Stacked on #224, which downloads the Bitcoin Core release source and authenticates it against the signed SHA256SUMS.

The first-install Welcome dialog repeats Bitcoin Core's own first-run Intro dialog, but as a hand-kept copy. This builds it from Core's strings in that authenticated source instead, so it follows the release CipherStick installs.

What changes

• install-core extracts src/qt/forms/intro.ui and src/qt/intro.cpp from the verified tarball, next to the files core: Authenticate pruning metadata #224 already extracts.
• New core-welcome-text reads Core's Intro strings, fills in Core's placeholders (client name, sizes, 2009), and keeps CipherStick's wording in three places:
  • "When you click OK, Bitcoin Core will begin to download" becomes "Bitcoin Core has begun to download";
  • "in this directory" becomes "in your Persistent Storage";
  • "If you have chosen to limit block chain storage (pruning)," becomes "CipherStick has chosen to limit block chain storage (pruning) to N GB (sufficient to restore backups N days old),".
• Every paragraph is kept; the text is not shortened. With today's Core strings the generated dialog is identical to the built-in one.
• Fallback: if a file is missing or any string was reworded upstream, the helper prints nothing and install-core shows its existing built-in text unchanged.

Resulting text (pruned)

Welcome to Bitcoin Core.

Bitcoin Core will download and store a copy of the Bitcoin block chain. Approximately 16 GB of data will be stored in your Persistent Storage.

Bitcoin Core has begun to download and process the full Bitcoin block chain (897 GB) starting with the earliest transactions in 2009 when Bitcoin initially launched.

This initial synchronisation is very demanding, and may expose hardware problems with your computer that had previously gone unnoticed. Each time you run Bitcoin Core, it will continue downloading where it left off.

CipherStick has chosen to limit block chain storage (pruning) to 2 GB (sufficient to restore backups 6 days old), the historical data must still be downloaded and processed, but will be deleted afterward to keep your disk usage low.

Not included: translations. Core's .ts files are only partly translated, which would give a mixed-language dialog.

Testing

• tests/core-welcome-text.sh (new) passes. It uses small excerpts of Core's Intro strings as fixtures and covers pruned and unpruned text, singular "1 day", reworded upstream strings, missing files and bad input.
• Same output from Core's intro.ui/intro.cpp at master, v28.0 and v26.0.
• Extraction paths checked against the real bitcoin-28.0.tar.gz, verified with its SHA256SUMS.
• shellcheck with CI's flags and bash -n are clean.

Refs #302

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https://claude.ai/code/session_01LvEHMDeHvYQYDEkpJn95X8

[BenWestgate]

github-advanced-security fails here because of a repository setting, not because of this change. It fails the same way on every open PR, including ones that touch none of these files, and the run reports no title, summary or findings. Nothing in this PR can fix it, so I'm ignoring it as instructed.

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[BenWestgate]

cACK

[BenWestgate]

aNACK, this should not add LOC of to the project to grab the Core text. Prefer our own dialog and not building it from Core if it can't be done in fewer normal length readable LOC than presently.

[BenWestgate]

Understood. It can't be done in fewer lines than now.

The built-in text is about 10 lines in install-core. Building it from Core needs, at minimum:

• extracting intro.ui and intro.cpp from the tarball;
• one lookup per string (six of them);
• the three wording swaps CipherStick needs;
• a fallback to the built-in text, which has to stay anyway.

Even trimmed to the bone that's more lines than the text it replaces. This PR adds about 70 lines plus a test.

Closing this in favour of keeping CipherStick's own dialog. #301 already does that and only adds the width fix.

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