askrene/downgrade/offers: dead code and doc cleanups from #9150

[nGoline]

Cosmetic follow-ups from #9150, none functional.

Dead code

• tools/lightning-downgrade.c:56,133,151: both wrappers pass convert_impression = true, so the if (!convert_impression) copy_data(...) branch is unreachable. The name also reads backwards, true means drop, not convert. Drop the parameter or rename to drop_impressions.
• lightningd/offer.c:380,451: prev_basetime is no longer assigned after prev_payment() was removed, so if (prev_basetime) json_add_u64(response, "previous_basetime", ...) is dead.
• plugins/askrene/datastore_wire.c: fromwire_dstore_channel_impression() takes const tal_t *ctx and never uses it, unlike fromwire_dstore_channel_bias() which needs it for the string. Drop it or mark UNUSED.

Robustness

• plugins/askrene/layer.c:1214: layer_trim_constraints() counts old entries with continue, but relies on the array being timestamp-sorted, then does tal_arr_remove_range(&intelarr, 0, count_old). If the array is ever unsorted the two disagree, and entries already stolen to tmpctx stay referenced from the array. break matches the documented assumption and fails safe.
• plugins/libplugin.c:346: json_id() blanks method when prefix needs escaping, leaving the unsafe prefix in the id. Unreachable today, every id a plugin sees is lightningd-generated and sanitized, and method is a compile-time literal at every call site. Fix is prefix = "!weird!".
• plugins/offers_invreq_hook.c:793: until = *offer_absolute_expiry - *invoice_created_at underflows if the offer already expired. Safe only because listoffers_done rejects expired offers first, which is an ordering assumption. A > guard on the subtraction, plus clamping the result to at least 1, would remove the dependency. Relevant because createinvoice_error treats invoice_relative_expiry == 0 as "cancelled".

Style

• plugins/askrene/layer.c:1142,1144: AMOUNT_MSAT(-1ULL), use AMOUNT_MSAT(UINT64_MAX) as add_constraint() does.
• plugins/askrene/layer.c:1223: if(count_old){.
• plugins/askrene/layer.c:1142,1144: AMOUNT_MSAT(-1ULL), use AMOUNT_MSAT(UINT64_MAX) as add_constraint() does.
• plugins/askrene/layer.c:1223: if(count_old){.
• common/utils.c:150: // p is a pointer-to-pointer for tal_resize. uses //, codebase uses /* */. The tal_arr_remove_range header doc says "Remove a range of element".

Docs

• doc/schemas/askrene-age.json: num_removed is described as "The number of constraints removed from layer", it now counts impressions too.
• doc/schemas/askrene-listlayers.json, askrene-create-layer.json, askrene-inform-channel.json: impressions[].timestamp description is copy-pasted as "The UNIX timestamp when this constraint was created."