Fix Toml.parse accepting tables defined twice - #9904

TOML 1.0 does not allow a table to be defined twice. Toml.parse accepted such documents and merged both definitions:

Toml.parse(`[fruit]
apple.color = "red"
[fruit.apple]
texture = "smooth"`)
// before: Success { fruit: { apple: { color: "red", texture: "smooth" } } }
// after:  Failure "Cannot redefine table 'fruit.apple' at line 3, column 14"

Dotted keys define each table on their path, so a header can no longer open one of those tables again. Sub-tables such as [fruit.apple.texture] still parse. Dotted keys can no longer add to a table that has its own header ([a.b], then b.c = 1 under [a]), and a [table] header can no longer name an array of tables ([[a]], then [a]).

The first test's document opened [database] after a root-level database.connection.timeout = 30, which is one of the cases now rejected. The dotted key moves under [database], and the parsed result is the same.

Against the toml-test 1.0 file list, the parser now rejects 10 more invalid files and still parses every valid file it parsed before. On 70,000 generated documents built from headers, array headers and dotted keys, its verdicts and parsed values match Python's tomllib on every one. Before, it accepted 2,659 of them that tomllib rejects.

Validation: pnpm test --run packages/effect/test/encoding/Toml.test.ts passes (19 tests) and the new test fails without the fix. pnpm lint and pnpm check pass.

Closes EFF-2907