feat(config): add source file tracking for config options

[m-mcgowan]

Summary

• Add get_source(section, option) method to ProjectConfigBase that returns the file path where a given config option was last defined
• Enables tools to determine which config file (main platformio.ini or extra_configs) contributes each option
• Useful for lockfile generation, config debugging, and IDE tooling

How it works

During read(), each file is independently parsed to discover what sections/options it defines. A _source_map dict tracks (section, option) -> filepath, with later files overriding earlier ones -- matching the existing configparser merge semantics.

config = ProjectConfig("platformio.ini")
# Returns the file where lib_deps was last defined
source = config.get_source("env:myenv", "lib_deps")
# e.g. "/path/to/extra_envs.ini"

Test plan

• Single file: all options trace back to the main ini
• Extra configs: options defined in extra files trace to those files
• Override semantics: when extra_debug.ini overrides a value from platformio.ini, get_source() returns extra_debug.ini
• Unknown options: returns None for nonexistent section/option pairs
• parse_extra=False: source tracking works without loading extra configs
• All 25 existing tests continue to pass (1 skipped -- Windows-only)
• pylint 10/10, black + isort clean

[m-mcgowan]

Follow-up commit adds a --show-source flag to pio project config that uses get_source() to annotate each option with the config file where it was defined.

Example output

Project with extra_configs pointing to shared config files:

$ pio project config --show-source

Computed project configuration for /path/to/project
platformio
----------
src_dir        =  /path/to/project/src  ; from platformio.ini
default_envs   =  esp32s3-idf           ; from platformio.ini
extra_configs  =  ../shared/platformio/platform_base.ini     ; from platformio.ini
                  ../shared/platformio/exceptions_base.ini
                  ../shared/platformio/idf_base.ini

env
---
custom_doctest_deps  =  https://github.com/eranpeer/FakeIt  ; from platformio.ini
                        doctest/doctest@^2.4.11

base:esp32s3
------------
platform                  =  https://...platform-espressif32.zip  ; from platform_base.ini
board                     =  esp32-s3-devkitm-1                   ; from platform_base.ini
framework                 =  arduino                              ; from platform_base.ini
board_build.psram         =  disabled                             ; from platform_base.ini
board_upload.flash_size   =  16MB                                 ; from platform_base.ini
lib_ldf_mode              =  chain                                ; from platform_base.ini
lib_compat_mode           =  strict                               ; from platform_base.ini

base:exceptions
---------------
build_flags    =  -fexceptions     ; from exceptions_base.ini
build_unflags  =  -fno-exceptions  ; from exceptions_base.ini

base:esp32s3-idf
----------------
extends    =  base:esp32s3    ; from idf_base.ini
framework  =  arduino, espidf ; from idf_base.ini

env:esp32s3-idf
---------------
extends      =  base:esp32s3-idf  ; from platformio.ini
                base:littlefs
                base:doctest
build_flags  =  -std=gnu++23      ; from platformio.ini
                -Wno-deprecated-declarations
                ...

Options inherited via extends show no source annotation (they're resolved from another section at query time, not defined in the consuming section).