Derive the lexer position lazily for seekable input

The lexer counted the read position on every character: two increments
and a newline check per get(), all on members kept in memory. For
seekable input (pointers, strings, other random-access byte ranges) the
position can be derived instead: chars_read_total is the adapter's read
count (including reads at EOF) minus a pending unget, and line and
column are recomputed from the consumed input only when get_position()
is called, i.e. on parse errors. JSON_DIAGNOSTIC_POSITIONS uses the new
O(1) get_chars_read_total(). Stream, file, wide-string and user-defined
adapters keep the eager counters.

Parsing pretty-printed input gets 8-12% faster and accept() 12-27%
(canada, citm_catalog, twitter; clang 21 arm64 and GCC 13 x86-64).
Error messages, byte offsets, SAX positions and diagnostic positions are
unchanged (differential comparison against develop on 548k inputs).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/home/architecture.md

@@ -185,7 +185,9 @@ The lexer detects two optional extensions at compile time. Only `iterator_input_
 random-access input of single-byte characters:
 
 - `supports_seek`, `get_consumed_count()`, and `copy_consumed_range()` let the lexer reconstruct already consumed input
-  for error messages instead of copying every character it reads.
+  for error messages instead of copying every character it reads. With `get_read_count()` and
+  `count_consumed_lines()`, the lexer also derives the read position from the adapter and counts lines only when an
+  error needs them, instead of counting characters and lines on every read.
 - `supports_bulk_scan`, `bulk_data()`, `bulk_remaining()`, and `bulk_skip()` let the lexer scan strings directly in
   contiguous memory, several bytes at a time.