refactor(transaction): replace sentinel head node with is_default flag (v14)

[satwareAG-ironMike]

Background

The tr_list sentinel head node pattern (documented in PR #553, issues #523/#541) is a design debt that should be refactored in v14.

Currently, index 0 of tr_list is reserved for the default transaction via a placeholder node (trans=NULL). The destruction path (_php_fbird_commit_link) uses positional i==0 logic to distinguish default vs explicit transactions.

This is fragile because:

• The positional convention is implicit (no compiler enforcement)
• 5 allocation sites must remember to create the placeholder
• The destruction path must remember to check i==0
• Removing the placeholder (as PR fix: #524+#523 - dead condition in blob_echo + spurious list node in transaction registration #537 attempted) causes use-after-free

Proposed refactor: Option B

Add an explicit is_default flag to fbird_tr_list:

typedef struct tr_list {
    fbird_transaction *trans;
    bool is_default;   /* replaces the positional i==0 convention */
    struct tr_list *next;
} fbird_tr_list;

Then:

• Remove all 5 placeholder allocations
• _php_fbird_commit_link(): replace if (i == 0) with if (p->is_default)
• _php_fbird_def_trans(): walk list looking for is_default == true (or allocate new default node)
• _php_fbird_trans_end(): replace tr_list->trans == NULL with "no node with is_default == true"
• fbird_inspection.c: replace *l == tr_list (first node) with (*l)->is_default

Files to touch

File	Lines	Change
php_fbird_includes.h:133-136	struct definition	Add is_default field
fbird_connection.c:86-128	_php_fbird_commit_link	Replace i==0 with is_default
fbird_inspection.c:236-245	transaction cleanup	Replace first-node check with is_default
fbird_transaction.c:954-1018	_php_fbird_def_trans	Walk list for is_default
fbird_transaction.c:1037,1070,1079	_php_fbird_trans_end	Update default-tx check
fbird_query_prepare.c:235-236	autocommit cleanup	Replace tr_list->trans == ... with is_default check
firebird.c:1500-1504	fbird_reconnect_transaction	Remove placeholder
fbird_transaction.c:367-372	fbird_trans_start	Remove placeholder
fbird_transaction.c:937-941	fbird_trans multi-link	Remove placeholder
fbird_transaction.c:963-966	_php_fbird_def_trans	Remove placeholder
fbird_query_exec.c:190-194	_php_fbird_exec	Remove placeholder

Acceptance criteria

• All 5 placeholder allocations removed
• is_default flag added to fbird_tr_list struct
• All 6 dependent sites updated to use is_default instead of positional logic
• tests/transaction_default_slot.phpt still passes (regression check)
• tests/issue119.phpt still passes
• tests/issue307_trans_start_nullable_options.phpt still passes
• CI green on all 12 jobs (PHP 8.2-8.5 x FB 3/4/5)
• No new ASan/LSan/TSan warnings

References

• PR docs(transaction): #523/#541 - document sentinel head pattern + regression test #553 (Option A: document + test)
• Issues bug(transaction): fbird_reconnect_transaction() allocates spurious empty list node #523, investigate: default transaction placeholder node pattern (from #523) #541 (investigation, closed)
• PR fix: #524+#523 - dead condition in blob_echo + spurious list node in transaction registration #537 (failed removal attempt, reverted)