bug: the catalog's own rules are not enforced — reserved names accepted, DDL text unnormalized, `DROP VIEW` missing

[dpsiderius]

Three gaps in the same CREATE/DROP dispatch and sqlite_master-writing
area that PR #717 just worked in. One PR closes all three.

3a — CREATE TABLE sqlite_master(...) succeeds and leaves a file stock
SQLite cannot open. SQLite reserves the whole sqlite_ prefix.

$ sqlite-rs exec m.db "CREATE TABLE t(a)"
$ sqlite-rs exec m.db "CREATE TABLE sqlite_master(a)"      # rc=0, no error
$ sqlite3 m.db "PRAGMA integrity_check;"
malformed database schema (sqlite_master) - table sqlite_master already exists (11)

oracle, same statement: "object name reserved for internal use: sqlite_master"

name	oracle	ours
sqlite_master	rejected, reserved	accepted → file malformed
sqlite_schema	rejected, reserved	accepted → file malformed
sqlite_sequence	rejected, reserved	accepted, integrity_check still ok

Pre-existing; #707 (PR #719) does not cause it (CREATE TABLE never went
through resolve_from_table_schema) but raises the stakes now that
sqlite_master is readable.

3b — catalog writes are refused for the wrong reason. Oracle says
table sqlite_master may not be modified for INSERT/UPDATE/DELETE and
... may not be dropped for DROP; we say no such table: sqlite_master. Right
decision, wrong explanation.

3c — stored DDL text keeps IF NOT EXISTS where SQLite strips it.

$ sqlite-rs exec m.db "CREATE TABLE IF NOT EXISTS t (a TEXT)"
$ sqlite3 m.db "SELECT sql FROM sqlite_master WHERE name='t';"
CREATE TABLE IF NOT EXISTS t (a TEXT)      # ours
CREATE TABLE t (a TEXT)                    # oracle

The stored sql text is part of the on-disk format, so this is a
byte-compatibility divergence, not cosmetics: anything reading schema text — a
migration tool, a diff against a reference database, .schema — sees a
different string.

3d — DROP VIEW is unimplemented. Any form falls through to the
unrecognised-statement path, so a view can be created and never removed.

Scope: reject CREATE TABLE/INDEX/VIEW whose name starts with
sqlite_ (case-insensitive) with the oracle's message, including the guarded
IF NOT EXISTS form; give catalog-write and catalog-drop refusals the oracle's
wording; normalise the persisted sql text as SQLite does (check what else it
normalises — whitespace, TEMP/TEMPORARY — and oracle-diff the stored text
across a spread of DDL shapes); implement DROP VIEW [IF EXISTS] (remove the
sqlite_master row, no b-tree to free, schema cookie bumps exactly once,
DROP VIEW naming a table errors like the oracle).

Complexity

Estimate: medium
Reasoning: Each part is small and they share one code area and one
test file; the care is in the test matrix, not the logic.

Refs: #697, #707, #717, #719