docs: make every docstring bullet parse under the house grammar

[edu-stark]

Why

The starkbank package's docstrings are the machine-readable reference the other 8 SDKs, the app-docs sample guard, and the sdk-c drift tables are meant to consume. A grammar-aware reader expects every parameter bullet to match - <name> [<type>>]: <description>. 217 bullets (of which 192 were genuinely missing the colon; the other 25 already had it but tripped a naive nested-bracket regex — see Verification below) were silently dropped by such a reader, taking 38 tags and 10 rules parameters with them across dozens of classes.

This is PR 1 of a 6-PR plan (see the AST-compatibility assessment) to make sdk-python a clean, generatable reference surface. Scope here is docstring text only — no code, no type-vocabulary changes (PR 2), no bullet reordering (PR 5).

What changed

• (a) Colon fixes — 192 bullets across 48 files were missing the : after the closing type bracket (mostly after/before date-filter bullets, plus one external_id and one user parameter). Fixed by inserting the colon; descriptions and wording otherwise untouched.
• (b) starkbank/corporaterule/__corporaterule.py — the 3 single-# section headings (# Parameters (required):, # Parameters (optional):, # Attributes (expanded return-only):) become the house ## forms (the last one becomes ## Attributes (return-only):, matching every other class in the package).
• (c) merchantcard/merchantpurchase comma-grouped bullets — every bullet that packed multiple field names into one line (- id, ending, funding_type, holder_name, network [string], - expiration, created, updated [datetime], and MerchantPurchase's card_expiration, card_number, ... / billing_country_code, billing_city, ... groups) is now one bullet per field, same type tag and text as the original group.
• (d) starkbank/merchantsession/__purchase.py — the Purchase class had no docstring at all despite 32 __init__ parameters. Added full ## Parameters (required/optional) / ## Attributes (return-only) sections; types and descriptions sourced from the "Create Merchant Session Purchase" endpoint and object definition in app-docs's merchant-session.json/merchant-purchase.json, and from the sibling MerchantPurchase docstring where the API reference didn't cover a field. One field (holder_id) has no source in either reference or a usable sibling description — it's documented with a neutral tag/description and flagged for follow-up (see PR description's Known gaps below).

CHANGELOG: no new entry added — ## [Unreleased] ### Fixed - Docstrings already exists (added by #168 today) and already covers this.

Known gaps (intentionally not fixed here, out of PR 1's scope)

• 25 pre-existing - name [type, default []]: ... bullets (list/dict empty-default type tags) trip a naive nested-bracket colon-check even though they already have a colon — noted so nobody "fixes" them into worse wording later.
• DynamicBrcode's slash-grouped bullets (id / uuid, updated / created) and the fully undocumented AllowedInstallment / dynamicbrcode.rule.Rule sub-resources — not named in this PR's scope (only merchantcard/merchantpurchase comma-groups and merchantsession/__purchase.py were).
• Purchase.holder_id's real semantics — needs an owner of the merchant-session API contract to fill in.

Verification

• grep -rEn '^\s*- [a-zA-Z_]+ \[[^]]*\][^:]' starkbank → 25 lines, all pre-existing false positives from the regex's nested-bracket handling on default []/default {} tags (already colon-terminated, unchanged by this PR). Filtered to true colon-less lines → 0.
• tools/.../measure.py (stdlib ast, no imports) rerun over starkbank: 0 single-# section headings; bullet parse rate up from ~88% to 96.1% (662/689); the remaining unparsed bullets are the same nested-bracket false negative, not new defects.
• python3 -m compileall -q starkbank → exit 0.
• A tokenize/AST-based checker (same approach as PR docs: carry the business rules the API reference states #168) confirms every one of the 252 changed lines across these 50 files falls inside a docstring literal — no code touched.

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