feat(pp): UPDATE FROM via std.ch + nested-bracket fix in matchSegment

`UPDATE [FROM <alias>] [ON <key>] [RANDOM] REPLACE <f1> WITH <x1>
[, <fN> WITH <xN>]` becomes a preprocessor rewrite to a new RTL
primitive __dbUpdate. For each detail record, find the master
record with matching key (forward-walk if both sorted, full scan
when RANDOM) and apply the REPLACE clauses in master's context.

Same shape as harbour-core/src/rdd/dbupdat.prg. The REPLACE clauses
expand to comma-separated assignments inside one block —
`{|| _FIELD->total := del->amt, _FIELD->status := "OK" }` — using
the multi-pair `[, <fN> WITH <xN>]` optional-repeat that std.ch
already establishes for SUM and DEFAULT.

Five-specific tweak: ON <key> wraps as `{|| _FIELD-><key> }` rather
than Harbour's bare `<{key}>`. Five doesn't auto-resolve a bare
identifier in a code block to the current workarea's field, and the
UPDATE block must evaluate against both detail and master so an
explicit alias prefix won't do — _FIELD-> dispatches to whichever
area is selected at eval time, which is what's needed.

Wiring up UPDATE surfaced one further matchSegment gap that fell
out of the multi-pair `[REPLACE ... [, ...]]` shape:

  * matchSegment didn't handle nested `[...]` inside its body.
    `[REPLACE <f1> WITH <x1> [, <fN> WITH <xN>]]` gave the inner
    `[` as a literal token to match against the line, so even the
    single-pair `REPLACE total WITH del->amt` form failed and f1/x1
    came back empty. Now matchSegment runs the same repeat-loop on
    inner `[...]` blocks that the top-level matcher uses, with its
    own outer-tail computed from the segment tail past the inner
    `]`.

Parser cleanup: UPDATE removed from the IDENT-statement no-op switch.

Gates green:
  go test ./...      : PASS
  FiveSql2 SQL:1999  : 43/43
  Harbour compat     : 56/56

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

compiler/parser/parser.go

@@ -1155,8 +1155,7 @@ func (p *Parser) parseIdentStmt() ast.Stmt {
 	// rewritten by compiler/pp/std.ch into function calls before the
 	// parser sees them.
 	switch upper {
-	case "UPDATE",
-		"LABEL", "REPORT", "ACCEPT", "INPUT",
+	case "LABEL", "REPORT", "ACCEPT", "INPUT",
 		"RELEASE", "SAVE", "RESTORE",
 		"DIR", "STORE", "NOTE", "TEXT", "ENDTEXT",
 		"WITH", "CLEAR":