Detect grid tables and emit error on parse

[rundel]

Based on the discussion in #156

Very much a proof of concept / vibe coded state at the moment but was able to handle the basic test cases I threw at it.

Sample output:

Fixture 1: bare grid tables (no block quote)

gt_block_quote1.qmd:

+----+
| oh |
+----+

+------+--------+
| key  | val    |
+------+--------+
| a    | hello  |
+------+--------+

+-------------+
| ```bash     |
| ls -la      |
| ```         |
+-------------+

```
+----+
| oh |
+----+
```

Run:

cargo run --bin q2 -- render gt_block_quote/gt_block_quote1.qmd

Output:

Error: [Q-2-36] Grid tables are not supported
   ╭─[ gt_block_quote1.qmd:1:1 ]
   │
 1 │ ╭─▶ +----+
 2 │ │   | oh |
 3 │ ├─▶ +----+
   │ │
   │ ╰──────────── Grid tables aren't supported. Use a pipe table instead.
───╯

Error: [Q-2-36] Grid tables are not supported
   ╭─[ gt_block_quote1.qmd:5:1 ]
   │
 5 │ ╭─▶ +------+--------+
 6 │ │   | key  | val    |
 7 │ │   +------+--------+
 8 │ │   | a    | hello  |
 9 │ ├─▶ +------+--------+
   │ │
   │ ╰─────────────────────── Grid tables aren't supported. Use a pipe table instead.
───╯

Error: [Q-2-36] Grid tables are not supported
    ╭─[ gt_block_quote1.qmd:11:1 ]
    │
 11 │ ╭─▶ +-------------+
 12 │ │   | ```bash     |
 13 │ │   | ls -la      |
 14 │ │   | ```         |
 15 │ ├─▶ +-------------+
    │ │
    │ ╰───────────────────── Grid tables aren't supported. Use a pipe table instead.
────╯

(Note: the trailing fenced code block in the source — lines ~17–21 — is not flagged. Content inside ``` fences is treated as a code block, so +----+ / | oh | lines there don't trigger the grid-table detector. This is the correct behavior.)

---

Fixture 2: grid table inside a single block quote

gt_block_quote2.qmd:

> +----+
> | oh |
> +----+

Run:

cargo run --bin q2 -- render gt_block_quote/gt_block_quote2.qmd

Output:

Error: [Q-2-36] Grid tables are not supported
   ╭─[ gt_block_quote2.qmd:1:3 ]
   │
 1 │ ╭─▶ > +----+
 2 │ │   > | oh |
 3 │ ├─▶ > +----+
   │ │
   │ ╰────────────── Grid tables aren't supported. Use a pipe table instead.
───╯

---

Fixture 3: nested block quotes with intermediate non-grid lines

gt_block_quote3.qmd:

> > +----+
> > | oh |
> > +----+
> | pipe-table-now |
> |-|
> | no |
> > +----+
> > | no |
> > +----+

Run:

cargo run --bin q2 -- render gt_block_quote/gt_block_quote3.qmd

Output:

Error: [Q-2-36] Grid tables are not supported
   ╭─[ gt_block_quote3.qmd:1:5 ]
   │
 1 │ ╭─▶ > > +----+
 2 │ │   > > | oh |
 3 │ ├─▶ > > +----+
   │ │
   │ ╰──────────────── Grid tables aren't supported. Use a pipe table instead.
───╯

Error: [Q-2-36] Grid tables are not supported
   ╭─[ gt_block_quote3.qmd:7:5 ]
   │
 7 │ ╭─▶ > > +----+
 8 │ │   > > | no |
 9 │ ├─▶ > > +----+
   │ │
   │ ╰──────────────── Grid tables aren't supported. Use a pipe table instead.
───╯